Gfrktyl

There is no magic item retail in my world. Instead, common to rare items are often sold at special auctions curated by the Order of the Sphinx, a wizard guild. Typically, the items for sale at these auctions are old heirlooms from bankrupted lords, the spoils of war sold by the state for revenue, and the occasional relic unearthed by a lucky farmer tilling their field.

My campaign is set in a semi-backwater region that is not home to many magic users or magic items; it would not be uncommon for the monthly magic item auction to have nothing to bid on at all. But when a treasure does turn up, wealthy collectors and powerful wizards from far off lands bid by proxy.

I'm thinking a wealthy collector might give discretion to well-paid hirelings in major cities to seek out certain types of items, within a price range. Powerful mages, however, would likely use magic to participate remotely in these auctions. What type of magic could they use?

I am looking for RAW spells and magic items that could facilitate remote bidding. Two examples that come to mind are astral projection and sending stones, but I'd love to know other options too, for the sake of variety and flavor.

At some point I might homebrew an option, but I'd like to establish what the RAW baseline is first to avoid redundancy.

Bidders might be as far away as 5000 miles from the actual auction location.

Please consider the cost of your suggested remote bidding strategies. The items in question (again, only common to rare) are valued within the same ranges indicated on page 135 of the Dungeon Master's Guide, so between 50gp and 5,000gp.

There are two basic strategies for solving this problem that I see. First, brute magic. Second, auction design.

Magic

There are a few simple ways for a powerful spellcaster to participate in an auction in real-time at an arbitrary distance. The two best are probably teleport and telepathy.

With teleport, the Order of the Sphinx would provide Associated Objects with specific auction houses to various rich interested parties, who would then use the object to teleport out to the auction site to bid in person. This requires that the Order of the Sphinx has the ability to manufacture and deliver a large number of associated objects, as well as communicate auction times to interested parties.

With telepathy, the Order of the Sphinx would provide (again, at a cost) familiar individuals for the spellcasters to target and speak to in real-time during the auction. One way to do this would be to have the auctioneer themselves travel to each interested party in the time before the auction to meet them and provide a valid telepathy target, and then the auctioneer could telepathically send auction status and receive bids when the auction is taking place.

Project image would also allow spellcasters to attend and bid in real time, but can only work if the spellcaster is within 500 miles.

In general, I don't think sending is a workable solution for a typical auction. Sending requires a separate spell for each message sent, and thus needs a separate spell for each bid if a spellcaster wants to bid more than once.

Auction Design

Magic isn't the only way to solve this problem. The idea of people sending in bids so that they don't have to personally travel to an auctioneer is a problem that has been addressed by real-world auctions for a long, long time. Take a look at the Wikipedia list of auction types for a few ideas.

One possible solution is the sealed auction. In a sealed auction, all bidders give their bid secretly to the auctioneer, who then announces which bid won after all bids are received. This works very well with sending. The Order of the Sphinx finds an item, identifies it, sends a sending to each interested party, the interested parties respond with their bids, and the item is sold for whatever the highest price is.

Another similar solution is to combine auction types. A powerful spellcaster tells the Order of the Sphinx the kind of items they're willing to buy, and what they're willing to pay. If any such item is found, then a normal auction is held with the bidding starting at the reserve price indicated by the spellcaster. This way, the spellcaster doesn't need to involve themselves with such petty tasks as bidding on an auction themselves, but they still can obtain items by offering more than others can pay.

Your powerful spellcasters have several options for attending distant auctions, which each have their costs and benefits. However, I'll first comment that the options you presented are not particularly viable. Astral projection will only work if the auction takes place on the Astral Plane, which is rather unlikely. Sending stones can only be used once per day, which is ineffective for an auction. But there are plenty of options which do work well.

Project Image

If your bidder is within 500 miles of the auction, then they can cast project image, a 7th level spell which projects an illusory version of yourself to somewhere within 500 miles. You can use the illusion's senses and speak through it, almost as if you were actually there, and other people also see you as present. This spell has a duration of Concentration, up to 1 day, which is enough time to not only attend the auction but to also do talking and inspecting before and after the auction. This, I imagine, would be the preferred option for spellcasters who are within range.

Scrying and Sending

If your bidder is more distant, then combining scrying and sending is an option. Scrying is a 5th level spell which requires a 1000 gp focus, which is within the means of a powerful spellcaster, and it allows the spellcaster to see and hear a particular creature or location anywhere on the same plane of existence. This will allow the bidder to watch the auction in real time. When the bidder wishes to make a bid, they can cast sending (a 3rd level spell) to deliver the bid to the auctioneer.

While this option has unlimited range, it has some disadvantages. Scrying takes 10 minutes to cast and it has a duration of up to 10 minutes. This means you need to know beforehand with reasonable accuracy when the auction will start (this can be achieved with sending) and you will have problems if the auction lasts longer than 10 minutes. Additionally, you have to use a 3rd level spell slot or higher for every single bid you make, which may be inconvenient for a lengthy auction.

Teleport or Teleportation Circle

Powerful spellcasters don't have to settle for remote communications if they don't want to. They have the ability to travel anywhere they want instantaneously. Thus, physically attending a distant auction is an option available to them.

Teleport is a 7th level spell which allows the caster to transport themselves (and up to 8 associates) to any location on the same plane with which they are familiar or at least have a good description of. However, unless they have an object from the destination or there is a permanent teleportation circle nearby, there is a probability of arriving off target which is a risk which may or may not be acceptable.

Teleportation circle is a 5th level spell. It will only work if there is a permanent teleportation circle near the auction (what 'near' is depends on how far the bidder is willing to travel by non-teleporting means), and it consumes 50 gp of material components, but it carries no risk of mishaps and is available to mid-level casters.

Telepathy or Voice of the Chain Master

This option requires the caster to have a trusted hireling or associate physically attending the auction. Telepathy has unlimited range and a duration of 24 hours and it allows the caster and target to share words, images, sounds, etc. instantaneously. This will allow for the caster to observe all that is happening at the auction and be able to deliver instructions in real time to their associate who will make bids on their behalf. Note that telepathy can be cast at the target from any range.

A similar option available only to warlocks is the Voice of the Chain Master Invocation. If a warlock with this invocation sends their familiar to the auction (by any means), then the warlock can observe what the familiar observes and speak directly through the familiar. However, sending a familiar over a long distance may be non-trivial, although retrieving the familiar is as simple as dismissing it (action) and causing it to reappear (another action). One advantage of this option is that it is available at only 3rd level. This will likely only be used if the warlock is within a few hundred miles of the auction, close enough for a bird familiar to fly there in reasonable time.

Crystal Ball of Telepathy

This legendary magic item allows the bearer to cast scrying and communicate with telepathically creatures within 30 feet of the sensor. This comes with the time limits of scrying but avoids the costs of repeated castings of sending. A legendary magic item is a high price to pay for this luxury, but for an exceptionally powerful spellcaster it is a worthwhile investment.

Sending is a 3rd-level evocation with unlimited range, so if magic is common in your world, it might make sense for remote communication to take place via Sending. Maybe wizards provide Sending services for wealthy clients, or maybe one could have a lucrative business making scrolls of Sending.

Xanathar's Guide to Everything includes a rule for scribing scrolls. Crafting a scroll of Sending takes 1 workweek, 500 gp, a short piece of fine copper wire, and requires proficiency in Arcana. So RAW it's 20 gp per word. This means of communication would be reserved for the wealthy and powerful.

This seems to indicate that magical communication would be reserved for matters where a lot is at stake.

That said, the spell says that it can transport 25 words. But it doesn't specify a language, and it doesn't say how many bits of information could constitute a word. So without breaking the rules of the game, you could transport potentially very complicated instructions or entire treatises if you had a large enough vocabulary of sufficiently meaningful words. Perhaps wizards in your world could have come up with a language for communicating via Sending with extraordinary efficiency.