Never trade in with a dealer, guns or cars. Always sell them yourself. Otherwise, you have to pay the store to sell them for you at a profit plus their overhead. If you are okay with paying another to sell your stuff, then go for it. But, how quickly do you think you could sell your STG for $825-900? For a few hours work (listing ad, replying to emails, shipping it, etc.), you can make $75 to 150.
I do not begrudge a store for making a profit. It is expensive to run a shop. A decent retail location will run $2500 up to $10K, depending on the location. Then there is overhead cost: electricity, salespeople's pay, inventory, insurance, etc. I work for a landscape company and we operate on a very low overhead as we have no office or storage yard. It still cost us $1k a day to operate, 5 days a week. So, not including material (brick pavers, cement, etc.), we have to have $22K a month just to pay overhead. So while buying a gun at $750 and selling it at $1.1K sounds like alot of profit, I bet it ends up being less than 15% after all the overhead is paid. There are very low margins in firearms sales.
So while I do not begrudge a shop for making money, I will not trade in a gun or buy one for full retail. It is their job to make a profit and stay in business. But it is my job to keep my money and keep me in business as well. Same goes with paying taxes. The Gov't trys to take as much as possible and I try to pay as little as legally possible.