I want to sell liftable objects and animals without using direct sales. For now it is pretty much possible if I have fenced area with locked gate inside claimed area, so I can place here an object I want to sell, allow trespassing, theft and rummaging, and then I can sell a key from this gate on a barter stand. After this transaction I will have to change the lock because buyer will have his key after previous purchase and will be able to open gate next time I will sell something else. This is not a problem at my own claim, but if I want to do this at public market, I will not be allowed to change the lock because no one give to me vandalism permissions. This problem can be solved by using consumable key which can't be used to make copies. It opens the gate only once and crumbles. A glass copy of key, made from one or two pieces of glass, may serve well for this purpose.
Sorry if this has already been proposed, forum search doesn't want to search "key".