From an airline stand point I don't see any reason you couldn't give your seat to someone else. As far as trading in the middle of the flight, I have seen people attempt to do this and I don't hesitate to let them know that this is not permitted and if they insist on doing this they will be charged for another FC ticket upon arrival at their destination. It takes away from the integrity of First Class and insults those that paid or used upgrades to be there.