What have you attempted so far? What’s the architecture for your item handling?
However you design the system, make sure that the clients aren’t the ones handling item transactions/replications to avoid possible exploitation. Ideally you would have either of the two parties fire a remote event to the server with a key for the item they wish to trade, and the server asserts the player owns said item before firing back to both parties that it’s been added to their offer. You want to reference it with a dictionary/tables instead of instances as those may manipulable by the client.