I was in EB last night, and I had one game to trade, but I wanted to maximize the deal, so I picked out 2 games that I actually wanted, and the 5 cheapest games they had (old sports titles. 3 were $2.95 each, 2 were $3.99 each), and bought them, getting 30% off. I made sure they all had manuals, but I didn't even make him pull the discs (except on the ones I was keeping). I took the five cheap games I bought, and the one game I brought in, and did 2 separate trades of 3 games each in order to get $20 extra in credit (the sports games trade in for $.25 each, btw).
The clerk was pretty cool about it, and he said that it actually helped him boost his numbers for both preowned sales and trade-ins. Probably not worth the time unless you have games you want to trade in that aren't multiples of three, but you can get an extra couple of dollars credit.
Here is the math invlovled if one does not start out with a game to trade, but instead buys 6 used games. Figure 3 @ 2.95, and 3 @ 3.99, the total would be $20.82. Subtract 30% and it is $14.57. Add tax (9.25% for me) and it comes to $15.92. Trade them in, and get $21.50 in credit. A 35% gain. It works even better if you have credit with which to buy them in the first place, so there is no real out of pocket expense.
$2.95 preowned PS2 games: NCAA College Football 2k3, NCAA Football 2002, Madden NFL 2002.
$3.99 preowned PS2 games: NCAA GameBreaker 2001, High Heat Baseball 2002, NBA Live 2001, NFL 2K3, ESPN NFL Prime Time 2002, NFL GameDay 2002, NFL 2K2.
$3.99 preowned Xbox games: NFL Fever 2002, Sega Sports NFL 2K2.
Don't bother with Cube, they are all too expensive, just stick with PS2 and Xbox.
Only do this if you have a lot more time than money (although going in with a list of the cheap games should help cut down on the time it takes). Make sure that all the games have manuals and case, or they can deny the trade, also no more than 1 of each title per trade, but if they have multiples of the $2.95 games, one can spread them out over multiple trades, potentially bumping the gain up to 58% or more, depending on tax rate.