In my (limited) experience of buying & selling engines, main dealers aren't that bothered with trade ins unless they have someone specifically looking for your engine. They are usually on a quota system from the manufacturers & have to shift X amount of new engines a year, so they'd rather sell new. Non-main agents might be more interested if you're buying a second hand engine that they have in stock. Not much help I'm afraid.
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5