If I wanted to sell a $50,000 mac pro, I sure as heck wouldn't trade it in to apple for $1000. I already know I couldn't even list it on amazon. I already know that the winners of auctions on ebay don't have to buy when they win, and people from foreign countries that you can't ship to can bid, even after you restrict bidding countries. I know that ebay buyers can pay you, wait for you to ship it, say they never got it, and ebay will refund their money. I would check with pawn shops and local computer stores, knowing they'd be unlikely to buy something so expensive and difficult to turn around. What would that leave me with? I don't know. A tax deductible donation? Scrap it for metal?
Everything that you "already know" is based on the weaknesses of the places you mention. These weaknesses are not natural, logical or eternal, they're man-made, and they ebb and flow over time. The basis of the whole thread was a valid criticism of how ridiculous the trade-in offers were. Good for you that you already know everything to be 100% static.