Gun shops are a business, and money makes the world go round and round. You have to make as much money you can from everything you sell. You have to sell alot of guns and related products to be able to pay your employees for all of the hours spent each day tending to the needs of tire kickers who end up buying very little, or nothing. Sure a shop makes money from a gun that is sold, but you still have to pay for you employees, and all of your daily expenses, even on the days that you don't sell any guns.
If you want to break even or not take a big loss, then sell it on EE, but if you are looking for the convenience of walking in with a old gun and walking out with a new gun then trade her in, if you don't mind taking a bit of a loss. When you figure in the amount of time spent by yourself to sell a gun on here, then you figure in your time spent packaging the gun up, and prepare it for shipping, then your time spend going to the post office, and your gas money, if you live out of town, then to trade in a gun for $100 less than you would sell it on EE, then its not such a bad loss, when you figure in your time spent selling the gun.