Now a gun dealer could probably be a little more precise but mark up on new gun is generally in the 25-50% range. A gun that cost you $300 cost the dealer $200-$225. No gun dealer in his right mind will give you what he can buy a new gun for or anything even close. From my experience guns we get for $300 have about a $150 trade value.
Obviously I don't have any of the specifics needed to give a firm answer but general rules of capitalism do apply.