Personally, if it were me, my money, and my desire for a semi auto shotgun, I'd honestly keep the pistol and buy a lightly used Remington. Heck, even a new one. You've got a nice pistol, accurately priced for trade value, and if you really don't want it, then I'd say it's a pretty fair trade straight up, or with some ammo on top. Can't see the actual page, work laptop blocks that access. The ATF import papers should have serial number, brand, date of purchase, but probably won't have date of manufacture. But if he purchased it a decade ago, you know it's at least that old.
Personally, I wouldn't make that trade. It might be a $600 value, it might be a $250 value. Hard to tell sight unseen.
One Oz. is correct as usual. Turkish guns aren't bad by any stretch, but you will get what you pay for. Also, one thing to consider, if this was imported and brought home by a serviceman, it won't have a manufacturers warranty. Whoever happens to import them today, if anyone, would not be obligated to do service work on that gun. So if you do the trade, hope it never breaks because you'll be hard pressed to find parts.
$500 will get you a used Remington 1100 all day.