You can set up player shops now (thanks be to the authour of iConomy).
See /warp mntc for an example of a working shop, or /spawn for a non-working but valid shop.
From the instructions:
Creating shop sign
In-game create a sign, on the second line you must but only “buy” or “sell” without the parenthesis. On the third line you put the itemID and the amount for every transaction. On the fourth line you put the price of each transaction.
It will look like so:
buy 1 10 10
Click “Done”, and it should create the shop, you will now be buying 10 (stone) at 10 coin per transaction. Replace “buy” with “sell” for a selling sign.
To show users how much you currently have in stock, place a sign anywhere around the base sign. must be touching the base sign however and it will update that sign with stock information.
Using shop sign
Go up to the sign, if you are the owner you can add stock by clicking the “sell” sign with the item in your inventory to add to the stock. Make sure that you have nothing in your hand, click the sign (left) and it will either a) tell you an error or b) give you the item or sell it depending on the sign you are clicking.