A FlyerTalk Posting Legend
Join Date: Sep 2002
Location: LAX/TPE
Programs: United 1K, JAL Sapphire, SPG Lifetime Platinum, National Executive Elite, Hertz PC, Avis PC
Posts: 42,211
No it is not the customer's seat to trade - the proper process is to notify an agent, request to be rebooked in coach so the next person on the upgrade list can be moved up.
I've witnessed this very thing onboard and have spoken up about it on the spot and was supported by the FA (who said "no") and my seatmate, also on the upgrade list with me.