Pass until you can figure both fair market for your gun and what a plastic SIG should go for. You don't think he's going to hurt with that deal, do you? Check the library for a copy of the Blue Book for a value.
Trades are probably the best money maker for a dealer. He can capitalize on the fact that you aren't sure what either your gun or his is worth and split the difference in negotiations and still come out smelling like a rose. Meanwhile, you will not be able to sit down for a week, so to speak.
