Double click or use the icons at the bottom to add items to your wishlist, then seek an item to find potential trading partners. More items on your wishlist will provide much better results. To make seeking easier you can sort by supply:

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Scout
Scout
Soldier
Soldier
Pyro
Pyro
Demoman
Demoman
Heavy
Heavy
Engineer
Engineer
Medic
Medic
Sniper
Sniper
Spy
Spy
Misc
Misc