There is no "official" percentage. A lot depends on (a) how much business you've done with the dealer, (b) exactly what it is you are selling, and (c) whether you are willing to do a consignment deal.
I'd say 20% is average. If you're a regular customer, and it's a gun that can be sold quickly, you might get by with 15%. And some dealers just won't do business with anyone who won't take a 25% cut.
On a consignment deal, where the dealer doesn't front you any money, you might be able to only pay them 12-15%. I've handled the estates of several MG-owning shooting buddies and only charged their heirs 10%, most of which goes to cover overhead (photography, creation of and placement of ads, etc.) as well as the time I spend answering emails and phone calls, taking additional photos, etc.
When you are selling a high-four or five-figure gun, you get a lot of tire-kickers ... and even the serious buyers can be nit-picky. Unless the dealer already has a customer looking for your exact model, selling MGs for full market value can be a time-consuming project, and meanwhile they've got their money tied up in your gun. Generally, they earn that markup.
Your Mileage May Vary.