You don't get 10% extra trade in at EB. Also it seems that when there is a special, like trade 4 get 40% off it seems that its just at GS. I dunno, maybe that will change now. It seems EBs have adopted the GS dvd sales.
Sell your games on eBay, you'll get at least double to triple what you would get for selling/trading in to those stores... I bought 24 last week for $40 and sold it on ebay yesterday for $35