Buying used games doesn't hurt a damn thing, unless of course you can come up with some proof that people buying a used copy of a game actually damaged said games sales.
That's some weird logic.
You have two people, two copies of a game. Person "A" buys a game new. Sells it back to game store at a later date. Person "B" comes along and buys that same game, used. The second copy sits on the shelves. The developers have been paid for two copies. Because of the constant cycle of games coming in and out the store always has this game in stock and does not need to order more.
Again, two people, two copies. Person "A" and "B" both buy a game new. Store runs out of stock and must order two more from developer. Developer has been paid for four games.
I am apparently not as fancy as most people. I buy most of my games used, because even five dollars cheaper is money in my pocket. I pay bills now, come on.
I often make a point to buy new for smaller scale developers, to help out. I do know used games hurt the company. Used sales are money to the secondary market, not to the developer. And although I support both, one of them put way more work into the product.