I'll try to start on a plugin like that, but i wouldn't get your hopes up, may take awhile, not gonna be home for next few days...
My own trade system, very similar to "Spout Trade", minus the spout: https://github.com/DarkCraft/DarkMisc/tree/master/src/main/java/net/darqy/DarkMisc/Trade If you can't figure out how to build it, I will for five bucks. Edit; err. SpoutTrade doesn't have anything Spout in it, anymore. Quickly checked.