Which store did you go to? After shopping around the Wilmington Gamestops, I've concluded that the stores in front of both the WalMarts (Market St. and the Monkey Junction one) have the best/most knowledgeable staffs. I tend to go to the Monkey Junction location because they're closer to my house and have hot chicks working some of the time.
But as a former manager, I can tell you that you probably dealt with a part timer or newbie in that transaction. GS also tends to run 1,001 trade in deals, further complicating the ability of someone who is there for three five hours shifts a week to keep on top of everything. For instance, in addition to the "trade in two games from this list" to get a game for $20 deal, pretty much all of those new release games qualify for a deal where your trade-ins get an extra 25% credit towards the reservation on it. Then there's the "trade in 3 games get an extra $10, four or more and get an extra $20" etc etc etc. They also send out weekly coupons in their emails and plenty of employees have never seen those when I bring one in.
But yeah, I recommend the Monkey Junction location next to the Quiznos. Nice big selection and a mostly competent staff.