I agree with Sheikah, there's no way this is the sole blame of the retailers. Otherwise many would try to undercut each other a little more than knocking off £2 for it being preowned. The fact is you've got newer and better games which have gone down in price in months.
At work I always notice Bioshock Infinite, which got masses of high scores (and awards?) now on 360 for just £25. And that came out this year. And this is just one example. And it sells, still.