Yeah, I like buying used games, and yeah, I'll buy them while the games are still in print, although I try to support games that I believe are going to sell poorly by buying a new copy. In the end, though, if I don't buy the used copy, the next person will.
I generally agree with Ian's summary of why used games (and anything else) are okay. If people aren't buying new games, maybe the games aren't good enough, or maybe the market is saturated? That's the cost of doing business.
It's also why a lot of products suck. When LED Christmas lights were first put out on the market, the claim is that they had thousands of hours of life. Basically, you'd never need to buy a bulb again. The following year, my sister's company started selling some LED Christmas lights which were under one of the popular Christmas light brands, and the bulbs were burning out right there in the store after a few hundred hours of use. Now, I shouldn't go blaming the corporation, it may be that the lights were never as good as advertised, but I can't help but wonder if the corporation purposely bought and crippled the technology, knowing that it would go out of business if someone was selling Christmas lights that could last for years and years without new bulbs.