If I am selling a newer gun I typically throw it into a auction of decent reputation when it comes to prices, and am typically pleased in the result, minus the sellers fee of course.
However, if I am selling a newer gun myself (gun show mostly), I would have to expect to get (and it actually sells first show) between 15%-35% below new value store price, with the least loss being at the lower price guns and the higher loss being at the higher end guns.
Keep in mind that newer guns are typically readably available, online and in store, and the only incentives for someone to buy a used newer gun is that it is less than the new store (as the internet can show anyone the current new market price), or it can be to a lesser degree that it is for sale right there and they can get it shipped to their door or it is already at the gun show they are attending, or it being that it is a hard to get gun.