As someone who used to work in one, I can't imagine anything has changed since way back then, let me try to explain how it works. Special deals and events aside every game has a specific trade in value. As near as myself and fellow employees could guess back then, this is based on several variables.
Time since release - obviously newer games net you more value. The most you are every going to get for a game is ~20-25 that you could get if you traded it back in the same week it came out.
Number of copies the store/company had - annual series games, for example Madden, you are unlikely to get almost anything for. We simply get hundreds of copies in trade.
Rarity of the title - independent of the number of copies there did seems to be a variable of rarity. You could bring in a bucket of PS1 games all of which trade in for less then a dollar probably, but if you had a copy of Vagrant Story or Valkyrie Profile in there then those might get $5.
Specific series - anything with Mario or Pokemon the system would give above average values too. Not sure why, it just did.
*if you want cash instead you loss a large percent of the value. It used to be 20%, I am not sure what it in now.
As other people have suggested the general rule of thumb is try to sell them yourself. Let say Gamestop buys your game for $5. They then sell it for $15. So instead you sell it to a friend for $10. Then you both make out with a better deal. You get more money and he gets it for less. The only time I have found it worth trading games in was in conjunction with specific trade in deals and promotions.
