Ending the game as soon as possible is pretty rational when you are so far behind you have no chance to come back.
What is a rational trade when you have no chance of winning? Ending the game as soon as possible is pretty rational when you are so far behind you have no chance to come back. If you want to avoid this, don't let people fall so far behind. You can do this by trading at only marginal benefit to yourself and large benefit to them. Normally you would only swap 1:1 if its going to let you gain a VP next turn, but if someone is miles behind, you might swap 1:1 with someone just to gain a slightly better hand, or even 2:1 to avoid having 8 cards for the robber.