Really tempted to trade in Metroid Prime as well even if I could get more on ebay. I do not think I want to deal with trying to sell it on there. I only paid $10 for it from Best Buy. Never played it and I know I never will.
You shouldn't have to deal with eBay. I'm sure there are plenty of CAGs here that would buy it off you for $40. And cash is always better than GS credit.