Quite some time ago, I remember reading an article that a guy put together where he took some of his most valuable games and did some price comparisons on what he could actually get for selling them. I don't remember much about the article, but I do specifically recall he took some of them to a Pawn shop who pretty much offered him complete crap.
Anyway, I want someone to do this again - take 10 or so of your most precious games (do a mix of systems) and see what you can get from various places. Check out some locally owned gaming stores (if you have any), a Pawn shop or two, then go on to GameStop, Walmart, Best Buy, Target, then go on eBay, search the completed auctions and find the auctions that are closest to your games and see what they sold for - but remember to take out the cost of selling on eBay, the PayPal fees and the shipping fees.
I'd love to see an article put together featuring a mix of older games and newer games (and I'm aware that the Trade-In-Value for, say, Chrono Trigger on the SNES is going to be $0 at the big box stores)...