If you bring them to Gamestop within a couple months of release, you can easily get $20-30 for them. I wish I didn't have to, as I often get urges to play some old game or another, but in order for me to be able to afford more than a couple new games a year it's necessary.
There's nothing ethically wrong with it btw, it's no different than sharing games with a friend. No creators of any other product in the world ***** about sharing as much as video game developers/publishers.
(Jeez, the captcha is almost impossible... nearly every one had lines through I/l/t/f or accents....)