I have a suggestion on how to make this happen. you can make a normal shop that takes out a set % for the final sell. this would pay for the storage. The shops would look like the others but it should be in a diffrent building and called a Market or a Flea market or someting like that. That way we know where we are going to get the stuff. and when the Purches is done the owner of the Items should be notified (with the option to squlech it if she/he wants to) and have teh money trasnfered to the user's accoun.
A way to balance this is also to charge the play the number of slots. make a flat rate for x number of slots and for every addtional slot should cost X amount ( that would stop it from being an extra bank. Or it should anyway)