when you sell a gun to a dealer, your expectation should not begin with 'what I paid for it'. the realistic idea is to see what a used gun in SIMILAR CONDITION (not perfect condition, if it is not perfect) is SELLING FOR now. The gunstore does not want an item sitting on the shelf forever, it wants to turn out volume by pricing to sell. Then, you must figure in the price they must buy at to make a profit. If your used gun is worth 400, they wont give you 400, or 395 (as many people expect). they will give you 280. why? multiply the used selling price x .7 . this is a pretty universal formula.
So, you bought a PTR 91 new for 1000, but used they sell for 675. You will be offered $473 for it.
on the surface it seems like you are being cheated, but really you are not. Also, that full trade amount should be tax exempt, which is a small consolation. This is why gunbroker is such a great thing, as you can sell for the going price direct to a buyer and eliminate the gunstore middleman....except for any transfer fees.
Like anything else, look for a deal and buy low, sell high when you can. But if you want a great deal on used guns, become a manager at a big chain store. I used to buy anything I wanted at our buying price plus a flat $50.
PS-sorry Ron, I just noticed that you posted a similar explanation