There is no incentive to buy new if the manufacturers themselves sell used.
So why sell used? Buying previously owned games doesn't mean you have to get in the market of selling them. Give people the option of returning their games for some amount ($10) or whatever that is credit towards another game. Heck, pay exactly what Gamestop is going to give them. But only if they bought the game new. So instead of all the money (and profit) going to Gamestop, the publishers get a percentage of the profit - and a relative guaruntee that the person will buy another one of their products.
Sure, they are effectively discounting their software, but only at the expense of what would have been resale units - which they wouldn't have seen any money from anyway.
Basically what it comes down to is that you can either try and eliminate the problem in one of two ways - making things more and more difficult for people who want to sell their games, or work with people who want to sell their games. A smart company would do that latter. I'm sure most of the current publishers will do the former.