I have a friend who owns a shop
He does not need the money generated by the shop. His father started the shop but passed away 5 years ago. The guy who owns this shop runs it more like a hobby than a business with most of his customers being friends. In my opinion he pays too much for his trade in and therefore has to charge too much for his used guns in order to make any profit. I look at it like this, If I am buying a like new used gun I want to save about 20% of the cost of a new gun. So, if the new gun that I am interested in costs $500 I should be able to buy a used one for $400 but I might pay $425. Once you get to the $450 dollar mark I would just rather buy a new gun, with all the goodies in the box, knowing that no one has messed with it. Assuming that the dealer needs to make a decent profit on that same gun he needs to buy it for between $300 and $325 dollars. So, if he buys it at $300 and sells it for $400 he makes a 33% mark-up and if he buys it for $325 his mark-up is about 24%. Most retailers, as I understand it, mark their products up 40%. Notice that I called the difference in cost to buy and price that the item is sold for Mark-up and not profit. Profit is the money left over after you pay all the costs associated with running the business.
So lets go back to your example, If the rifle that you buy cost $400 dollars originally and you get it for $300 then you have initially saved $100 or 25%. If you then turn around and sell it back to him in the same condition in which you bought it, and he gives you $200. Remember a used gun is a used gun is a used gun. The only thing that matters after it has been sold the first time is condition. So, if you had bought it new for $400 and he bought it back from you thinking that he could get $300 for it, how much is he going to pay you? He is probably going to give you $200, the exact same amount he would give you for the used one that you bought for $100 less, assuming that they are in the same condition of course. If you were to sell it on a website such as this or the others allready mentioned you might be able to recoup all of your money. You will rarely do that with a new gun unless you get really luck and find an unobtanium at a reasonable price like an AR-15 2 years ago.