It's not black and white. When talking about GA's I can understand that it may seem rude to trade a gift but unless the gifter has stated otherwise, the GA winner is now the owner of the code and can do whatever he want.
In trading it's not even rude imo. Let's say I give you a chicken for your cow. You get my chicken and you happily walk away, got what you want. I got my cow, I am happy as well. Is there any problem if I'l trade the cow for a pig with another person ?
Each of the two sides got what he want based. The basis of GOG is trust, and I don't think that this idea is broken unless stated otherwise and agreed upon both sides.
I'm not referring to trades, and the list is not about those either (if you had read any of the links you'd see it), though, imho, the honest thing is to say that one wants to trade something further.
It has happened with prizes from GAs that clearly stated the games were for personal use.
But even if not (the GA holder may have not thought that one would enter only to trade the prize), is it OK to enter the GA without stating that a prize is wanted for trading purposes? Or at least inquiring if it's OK? Entering a GA and saying how much one wants it, only to trade it for something else as soon as they get it, when others enter because they actually want to play the game, is it OK?
You spoke of trust, and trust requires involved parties to be upfront and honest, doesn't it?