You want to trade with a person -> Process sent to server.
Server verifies -> sends confirmation to you, and your "trade icon" spawns -> person you traded gets the message.
Person who got traded accepts the trade -> trade acceptance sent to server.
Server verifies -> Trade window opens for both players trading with each other.
Person A puts item in the trade window -> trade process sent to server.
Server verifies -> item is put into trade window -> person B sees the item now.
Person A clicks accept -> server verifies and shows accept trade to Person B -> Person B clicks accept -> Server verifies both trade acceptance -> Item is traded