That would be very simple to make. Would need to have a failsafe though, so people can't put stuff in it, if there isn't anything left to trade for, and keep track of payment, if they pay too little.
INSPIRATION! As my internet is borked atm, I might just start a little project. Been ages since I have coded anything in Java, only been doing HTML, CSS and JavaScript lately. Could use the change.
Could people specify some requirements? Should the currency be emeralds? That would make a very slow start, as there isn't much emeralds around atm. is there?