That’s a tough one to answer as there is no hard and fast rule for markups. Market demand, price paid, and competition all impact how much markup you can apply to a gun. A new gun bought from a distributor will have less of a markup than a used gun bought for pennies on the dollar at an estate sale.
Typically markups of 10-35 percent are common on new guns. Used guns may have a higher markup, or they may not if there is demand for it and the owner is unwilling to sell for much below common retail. Sometimes you can accept guns on consignment and take a fee for selling it which again changes on type of gun and market demand. Manufacturers will have a suggested retail price or even minimum price you can advertise, but they are all suggestions and you can adjust your markups based on what will turn you the most profit.