Opposable Thumbs looks into the high margin world of used video game sales at local retailers. From the article: "The only place that margin can be improved in [retail game stores] is through used games, so of course the stores give the least possible amount for trade-ins, and sell them for as high as they can. You don't know how brutal it can be until you give someone US$5 for a game, and then sticker it for $34.99 as they walk out."
Follow the link for the full article, and be sure to check out these free alternatives to trade-ins: Swaptree
(private beta), TGN
, and Game Trading Zone
. While not free, you'll still get market value on a trade-in on the Internet
, of course.