To give you a couple of data points:
I've brought them one gun for sale. If I had done a consignment sale there, they would have charged me 18% of the sale price, or roughly a 22% markup. The price Tom offered to buy it outright would have given them a 36% markup. Now in both cases I was looking to walk away with cash, not store credit. I didn't sell it. I've since come to my senses and decided to keep it.
If you're trading, not selling, you should be able to do a bit better than that. I've traded one gun there: I swapped a 637 with box and papers even for a pre-lock 642 with no box. They effectively made about 9% on the deal (based on their asking prices for the two guns). I think we both felt we got the better end of that deal.
The worst that can happen is they give you an offer you don't like. The shop's worth a visit even if you don't end up making a trade.