It's a tricky one..
In this economy, I generally don't buy full priced games unless they're really special.
I get them pre-owned if it's a popcorn game.. I buy brand new games sometimes, but these are the epic games I truly look forward to like Mass Effect, Dragon Age or Portal2. I don't feel guilty about getting a cheap 2nd hand game though.. They're normally old, the person who sold it doesn't play it anymore and I get to enjoy a game I wouldn't have purchased at full price and perhaps I'll fall in love with the franchise and get the sequel at on release day. As long as the pre-owned market is mega cheap, then it's OK.
What is a problem is when the pre-owned games are almost as expensive as the New games.. this does deprive the game maker of a real sale from someone with the cash to spare for a new full-priced title.
Tescos have started getting into the used games market and their used games are almost as expensive as the new games.. (last time I checked) that's totally not on.
If the game is mega cheap, then the person who buys it probably wouldn't have brought the game at full price so nothing is lost.
If it's almost as much as the new game, then the person is choosing the 2nd hand game to save a couple of quid and would normally buy a new copy - depriving the game maker.
So basically, it's only a lost sale if the full priced sale was there in the first place.
Businesses who re-sell 2nd hand games at near retail prices should be made to a) ensure all disks are of an immaculate quality and b) send money to the publisher if they wish to justify the cost. Otherwise, the customer and the publisher are being duped out of cash, the customer is getting a sub-standard product and this is considered legal.