I am trying to do a simple trade. My item for 2 keys. I get the error below. Any tips?

Shop overloaded. Trade occupies 2 slots but only 0 slots available.
New Trade
Sell
Lv.76
Track Terrorizer

Buy
Mann Co. Supply Crate Key
Mann Co. Supply Crate Key

Wishlist items pre-occupy the shop space with the current system (so that you won't have more than 12 items in your shop after you sell).