I just look at how long it will take me to finish the game. I've played some absolutely great games that were renters. Trauma Center 2nd Opinion was fantastic but I was done with it after a week. I was on the last level of Zack & Wiki when it was due back at the store. I'll rent it again and finish it when I'm bored of my other games. I don't think Marvel: Ultimate Alliance is as good as either of those games but it'll take long enough to finish that I was better off buying it at Gamestop for $18 used than I would've been renting it at $7 per week.
One thing Nintendo does pretty well with their in-house games is make them long enough to be buyers instead of renters. There's no way Zelda, Metroid, or Paper Mario are renters. It would be a lot cheaper just to buy the game.
In my opinion, most party games are usually better as renters. Just rent it when there are people over. That way you don't get better at it than everyone else. The only exception I ever make to that rule is Bomberman. I'll always buy Bomberman.
I know a few people who buy on eBay and sell on eBay. They usually end up losing about $10 for the couple of weeks they own the game through eBay listing costs, shipping, and PayPal credit card fees. I'm tempted to try that myself but I'm not authorizing my PayPal account until I get around to setting up a bank account that's "safe" for PayPal. I'm not letting them touch the same bank account I pay my bills through.
