The law of used guns indicates that they pretty much have to do that to stay afloat. No customer is going to pay retail or even Bluebook for an average used gun because of the risk of hassles, newness factor, changes in the market demand, etc. A small gunshop also risks that the used inventory does not sell and most shops do have to pay carrying costs such as insurance and property taxes on inventories. By the time you include your markup for overhead--rent, light time, carrying costs, a gunshop would not make much on tradeins at 60%--most small shops do their best when they sell a primary item that can be easily price shopped for a low markup but have accessories that are far more difficult to priceshop that are often sold at the point of purchase.
Pawnshops can often offer deals as they buy from those desperate for immediate cash or someone pawns a firearm and doesn't make the payments.