Okay... First of all, here's my policy on game buying:
1. A new IP that I want to give money to or a sequel to a franchise I love - I'll buy new.
2. A game that I'm leery about for any reason - I'll buy used once it drops to $40 or so.
First, to address the "evil corporations' trade-in policies"... The only retailer I really frequent for games in GameStop. Why? Their staff is always friendly and can actually answer the question "Have you heard anything about this game?" They tend to stock every new release - for consoles, at least - regardless of how obscure. I'm aware of just how little they'll usually offer for trade-ins, so I wait for the bonus credit incentives they always run. I've traded in relatively recent games that I just wasn't playing anymore during these offers and I've often gotten close to what I paid for them.
I never used to trade games when I was younger. I had a collection of somewhere over 100 NES games, 80 or so Sega Genesis games, about 100 PS2 games, and at least 50 Xbox 360 games. Then I moved into the real world where things like bills and rent are a concern. Now, given the choice between re-playing an old game that I'm sick of or trading it in to save for a new game that I want, the choice is obvious. There are some games (Fallout 3, Mass Effect, Dragon Age: Origins, fighting games) that I won't trade in because the replay value is still there for me. However, I don't really have the luxury of holding to games just for the sake of hoarding while still spending money on new games.
The used game market isn't hurting the gaming industry nearly as much as they'd like us to think it is. After all, the auto, music, and DVD industries have managed to survive the used market and I don't hear nearly as many complaints from them. Selling used video games is a valid form of distribution, and if the developers and publishers don't like it, they'll need to find creative ways to encourage consumers to buy products new. Looks at the often-maligned Catwoman DLC code for Arkham City; I actually think that's a fair tactic to use to encourage the purchase of a new copy of the game. Besides, someone can pick up a used copy someone down the line for $30, pay $10 extra for Catwoman if they really want, and they'll still get the whole experience for $20 less than I did on day one.