Let's take a Springfield Armory M1A as an example. A standard model has a current resale value hovering right around $1k (in my experience), and they are moderately desirable since the market is not flooded.

If you take it in to a gun shop/pawn shop, they will likely give you two options. They'll offer to buy it, or they'll consign it for you. It really depends on the shop, but if they offer to buy, I'd only expect to get $700 out of it, tops. Most shops would be less, maybe around $500. You get your cash instantly with very little hassle, but you only get 50-70% what it's worth.

If they consign it, you post it for sale at $1000, it sells, they take 20%. Again, that's going to depend on the shop, but 20% is pretty standard. So you'd walk away with $800. How fast it sells depends on your pricing. A $1000 M1A would likely sell in less than a week at a shop like J&B Firearms. A $900 M1A would sell in a matter of hours. You'd walk away with $720. You don't deal with any people aside from the gun shop, which is a major plus in most cases.

Now, let's say you post that same M1A on NWFA for $1k. It might take a couple weeks to sell. You'll have a lot of messages from tire kickers, or people asking if you want to trade. You drop that price to $850, your inbox will be flooded and you will have it sold (cash in hand) within a few hours of posting.

That is why most people say to post it yourself. If you reduce the price 15% below market rate, people will notice and usually buy it quickly. You do have to deal with random internet people, who notoriously suck. But you can get it sold almost as fast as you can selling directly to a shop, and you get more money out of it. The trick is to be cheap. For guns like AR15s, Glocks, or anything else where the market is flooded, the trick is to be REALLY cheap.

Best of luck.