Screw GC!
Ebay, flyer local music stores, or ads on local musicinas websites. if you have something worth $300 as store will give you $150 and resell it for the $300 it was worth. as a rule a store wants to make back double whatever they give you so at best you'll only get a fraction of what's its worth. And if the counter person is stupid you may get less than it's worth. I tried to trade 400w pa amp at GC and they said it wasn't worth anything.