When people go off to sell their precious used gun, most of the time it seems like they are convinced that they should get back what they paid to begin with. The problem with that is simply that no one is going to pay full price for anything that has been used. Even some gun shops will knock off the price of their guns if they are on display for a while and have minor signs of wear.
Think about the fact that a used car today does not give a good return, unless it is extremely rare and comes practically new with few miles on it. Generally, all guns are subject to this, but with some fluctuation, depending on how popular the gun is. Supply and demand will dictate how much you get for a used gun. For example, you could sell a well used M1 Garand for over $1000 because the supply is relatively low and the demand is high, which jacks up the price naturally. Unfortunately for the consumer, as soon as you buy that gun and walk out of that gun store, the price of the item you purchase may go down instantly and further decline with time.