There is no currency in Untold Lore, and nothing that says a sword is worth three fish. Trade is barter: two people, two offers, and an exchange both agree to — worth is whatever the two of you decide it is today.
The invitation
A trade starts as an invitation, and an invitation costs nothing to ignore — it fades on its own if the other side is not interested. Nothing opens on your screen because somebody else wanted it to; the window opens when you accept.
Escrow
An offered item leaves your pocket and sits in escrow — visible to both sides, owned by neither, and impossible to promise twice. When both sides commit, the offers swap whole; if either side walks away, everything returns to whoever offered it. There is no timing window in which a quick hand profits.