ACTUALLY it CAN be a good deal.
Only in the sense that if you use the trade in value as the last part of money owed, you don't have to pay tax. which removes about 19-26 dollars (Chicago tax is 10% WTF) from the purchase.
say, you are working up to getting a 3DS, and pay off 150. with a combination of cash, trade in credit, or whatever. You pay off that last bit with the 100 XL trade in, you SAVE money (equal to how much extra you get selling it online).
Keep this in mind. And if they SOMEHOW try to charge you tax anyways, don't let them. They are not supposed to.
I worked there and did this for some of my pre orders.