Of course they pay pennies on the dollar, what do you expect? In the old days (30+years ago) you sent them your junk gun, and sent you a check (or was I a money order?..been too long, I don't remember...
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If you didn't like the offer, you sent it back, and they sent back your junk gun. Numrich also used to be one of the last places that still shipped COD. I don't think they do that anymore...does anyone?
A wrecked car has probably at least a couple hundred dollars (maybe much more) of useable parts, but you won't get that value if you sell it to a wrecking/salvage yard.
Same with guns. You might have a gun with $150 worth of good parts in it, but you'll only get that much if you can advertise, and wait for each of the parts to sell. It might take months or longer just to for someone looking for a specific part to find YOU. And then, of course, they're going to want the part for dirt cheap.....
Computers make it lot easier than it used to be but its still not easy to get even close to full value from used parts. First of all, they're used parts, but mostly, selling, and shipping each piece, as it happens to sell (IF it does), can take time and effort, spread out in little pieces over a long time.
Numrich offers a little bit of cash, now, for what most people would only see as junk. IF its not enough, simply don't sell to them. What ever money you make, or costs you incur is entirely up to you.
Another thing to consider is what it costs them after they buy your parts gun. They have to strip it, inspect the parts, catalogue, and put them in stock. That takes time, and costs money. I'm sure that plays a part in their offer, too.