Nobody's coercing the customer into taking the trade.
I had an FFL back in the 80s and would setup at gun shows. People would sometimes show up at my table with a gun and ask what I'd give for it. I'd throw out a lowball number and 9 times out of 10 they took it. The 10th one might ask $10 more and I'd usually pay it.
In retrospect, I should've asked where they got it, but I never bought from anyone who looked seedy or suspicious. I think most just had a gun they'd found in grandpas closet and wanted to turn it into cash. I never poached any valuable collector pieces, just run of the mill sporting guns, mostly pump shotguns.
While I flipped those guns for a decent profit, I was also tying up precious capital for an indeterminate period, so the pay out had to be worthwhile.
Gun shops, especially smaller ones,have become a threatened species, so I won't begrudge them a profit.
While browsing one of my favorite family shops back in the day, a customer was complaining to the owner about the price of something and said "This is my hobby, I'm not gonna pay that much". The owner responded by saying "Well, it's my living, and I'm not gonna give it away."