Everything has Quality has a patch so all silver is priced the same in the normal trade menu, but it seems to act oddly with this interface
Specifically it only seems to happen if you have a different quality of silver, say Awful

Basically you select your items for trading, and then unselect them, and it'll add/subtract a range of silver from the deal on both sides, this allows you to do fun things like gain silver for no cost

I don't actually expect you to fix this, just posting it here so anyone else who runs into it knows about it