My experience is only as a buyer, as I have never sold a gun; however, I have relationships with many sellers who tell me about their experiences.
Gunbroker:
This is the mass selling platform. There's an art and science to selling well on GB. Frank Goepfert has perfected the art of selling on GB, and he does well, even though a lot of experienced MG buyers hate him.
Often, you'll get a really good price for your item on GB, but sometimes you'll get burned. You're thinking, 'I'll just set a reserve price to protect myself. Well, there's a cost to doing that, in that it can turn people off and it blunts momentum (because after people bid, they get emotionally invested in an item and will bid up to crazy prices; also once someone bids, even its a really low bid, GB will advertise for you by sending emails to people who have bid, which can't/doesn't happen if people never bid because there is a high starting price). I'm not a fan of the reserve price for that reason. If you want people to go crazy, you should be willing to walk without a safety net. The highest priced auctions, that I've seen, are the 1 penny, no reserve auctions. Frank uses those a lot, and then he sets a buy now price that's about 20% above market. Nowadays, someone always hits on the buy now.
If you look at the other MG GB auctions, where the starting price is at or close to the retail value, those auctions get the lowest average price, if they even sell at all.
To get the highest price on GB, you have to be a bit brave.
The biggest downside to GB is that there are lots of deadbeat buyers and tire kickers. Lots, from what I am told.
Sturm:
I've bought guns off of Sturm.
The downside of Sturm is that there are TONS of scammers on Sturm. The owner is always trying to take down and purge scammers. The new site layout is helping, but if you come in with zero or minimal credentials, you will be definitely scrutinized.
Based off of their forum activity, Sturm also has the lowest traffic volume of the major NFA sites. Lower volume means fewer visitors, which results in fewer buyers.
Subguns:
Kind of like Sturm, but more active. The hardcore guys tend to buy and sell off of Subguns. I haven't seen too many scammers try to scam on Subguns, but they're around.
I've purchased off of Subguns, and it's my favorite place to find hard to find items. Some really good stuff will pop up on Subguns.
Some other well known avenues...
Using a broker or buyer like Ruben:
Ruben has told me that he needs to make a certain threshold percentage. I won't disclose what it is, but it's on the higher side. In return though, he's reliable and provides a good service. Ruben only buys, he doesn't broker.
Frank Goepfert is a broker and buyer. There's plenty written about Frank. I've bought guns from Frank. I won't buy from Frank anymore; I won't say any more about that. Frank mainly sells all of his stuff one of three ways: 1. GB auction, 2. His massive email pre auction list; 3. He offers items to reputable buyers for sale.
Here's some advice if you want to sell for cash: insert the line, NO TRADES. Guys will try to trade you for stuff, and I am told that is quite annoying if you're a seller.
The biggest factor that determines the success of your sale is the price. Price, price, price. Price is way more important than the selling medium.
If you're trying to get a reasonable price for your item, the item will practically sell itself. In that case, you should go for the lowest commission/expense/cost.
If you want a higher than market price, it will take some effort and time. Key word here is time. I always get my best deals from sellers who, for whatever reason, are forced to sell immediately. A good deal isn't a slash and burn price; that's just taking advantage of people, and I'm opposed to that. A good deal is like 5% below market; for example, a good deal right now would be $15,000 for a RR AR or $25,000 for a naked Flemming HK sear. At those prices, you will sell in the same week. If you list the same RR at $14,000, you'll sell in the same day. At $13,000, you'll have a line of people tripping over themselves to buy the gun (assuming you can prove that you own the gun). At $12,000, you're probably a scammer.
Also, the item itself is very important. Certain MGs are very easy to sell, even if it's above market. There are certain items, which if posted for sale, a line will start to form for them. Since I am a buyer, I won't go into what these items are.
Certain items are not worth trying to sell. Used SBRs, SBS, DDs, and silencers are worth practically zero. Check out GB and you will see that all the SBRs, SBSs, and Silencers have zero bids, and many of those items are new. The easiest way to sell a SBR or SBS is to attach a title 1 conforming barrel, remove it from the registry, and then sell it as a title 1 firearm.
There are exceptions, of course, to what I said about the nonMG items I mentioned above. For example, an original HK MP5SD suppressor is worth a lot. A used LMT M203 won't sell, but a factory original Colt M203 is quite valuable.
Good luck selling your items.