Folks used to get highly irate when they would bring a gun to me to sell it back to the store. EVERYONE thinks that they should get back what they paid for it. EVERYONE. Even folks that know the business. Always amazes me and leaves me shaking my head.
This is how it's played out for me for the 30 years and different stores/ranges that I've worked:
Pretty much the standard quote is 1/2 of DISTRIBUTOR/WHOLESALE price (my price) - whatever % factor for wear and tear/condition. Our gun markup is generally +15% (15% is enough to keep the doors open and the lights on....barely).......so something that costs me $500 to put on the wall has a $575 price tag on it.
Call it a $600 pistol, out the door.......shoot a few boxes of ammunition through it, keep it for 6 months....then need cash for Christmas purchases.......trade-in for me to buy it? Yep, you guessed it......$250. They leave angry as hell......go to a pawn shop and get offered $125.....and come back to me a week later BEGGING for $350 or their old lady will leave them. That's when I put $250 cash and a bill of sale that they need to sign on the counter.....and 99.99% of them take the $250.
I then take that $250 pistol, with a few hundred rounds through it, send it to the gunsmith for a full detail clean for $35, and put it up on the wall as 'USED' for $450 right next to that new $575 one.........and sell the used one the next day.
At one point I audited our bound book and nearly 20% of our stock 'rotated' in this manner at least once before leaving forever. I had one Springfield Armory 1911 that I sold 4 times across 2 years before it never came back.
Want a nearly 100% predictor of that that 'rotating inventory' gun is going to be? Keep an eye on whatever gun the hero/villain uses in the latest action movie. Chances are that it'll be flying off of the shelves for the next month or so, only to come back in stacks the week after purchase, or the week after Thanksgiving when Christmas sales start. It's such a 'thing' that our distributor reps often call with 'movie specials' the week or two BEFORE a blockbuster movie comes out.
Trade in to buy something else? $300 STORE CREDIT ONLY that cannot be cashed out.
Same with rifles, except that any Franken AR that someone assembled from a lower and parts at home gets an auto minus of about 50%......even if you have the receipts to prove your high end parts.......If YOU built it, for me to re-sell it I have to disassemble it, clean it, and reassemble it, PROVING each part is what you say it is, if possible (and most parts are impossible to prove lineage in the AR world) and make sure it works......or I'm left with a dud that no 'manufacturer' will take on a warranty claim and I have to eat the cost of making it work........after all, it was sold as parts, NOT a complete rifle.
The harsh reality is, if you want more $$$? Go peddle it on ARFcom, I'll see you back here in a week.
As for bolt rifles, unless it's a limited run or a distributor special, no special consideration, even for accessories. Best bet is to sell those separately on ARFcom, Gun Broker, or Ebay, because my mark up on those is ABSURD, and you would be insulted by any offer I would make on them individually.