That would be really hard to do with ChestShop, and it would be unneeded as you can just set up 2 separate shops using money as the middle man.
You can possibly make a trading post with redstone. Just wait for clear lag and then put your items in a trading post. To make a trading post, I suggest looking for a tutorial video.
This is a good idea, although it would be hard to do. As stated above. It does reduce risk of getting scammed, though.