The idea of an auction house sounds super cool. I like the idea of the items being in a castle and the mob going to said castle and retrieving the item. This allows the item to be susceptible to theft yet still allows the idea of auctioning items as long as one wants. That is the whole reason to having the shopkeeper capable of going to the castle. I think it would be prudent to have this auctioneer un-able to get a scry on. That way people wouldst try to teleport to the auctioneer after one purchases an item and allowing that person to loot the auctioning person's castle/inn/treasury.