I've done quite a bit of Gunbrokering and Auction Arming, and I think I've learned a few things. The two best ways to make money with internet firearms are to: 1) buy locally and sell on the internet, or 2) buy online, and sell locally. Of course this depends on where you live. If you set up a free standing ffl, let's say a pawn shop, you've got access to the random firearms that folks want to get rid of pretty quick, and we all know that pawn shops and gun stores will give you about half of what the gun is worth. So, you buy the guns at 50% of blue book and sell them online at 75% of blue book.
The reverse also works. By guns online from the auctions; just keep a lookout for those that are going for way below their book value, then sell them locally at your shop or through local classifieds with a markup over what you paid. You'll be reaching the folks that would never purchase a gun directly off the internet, and offering them a lower price than your competing pawn shops and gun stores.
I think there's a few keys to your success, though. First is to go primarily with used guns. You cannot compete with the major distributors online with new gun prices. Second, KNOW WHAT GUNS WILL SELL FOR. Don't go by the Blue Book alone. Just familiarize yourself with the most popular stuff and know the market. Like, I know that pretty much any Glock is going to bring at least $400 online or in person. So, any one you could pick up for less than $325 is a buy. Any real M1 carbine in good shape will bring at least $575. Find one for less than $425-450, buy it. It also helps to kinda specialize in certain genres or brands, like you said. Right now, old model Ruger single actions are hot. Any one in .45LC, .44mag, or a convertible that you can get for less than $350 is a great buy for reselling. Basically you have to take the advantages of your two markets, local and the internet. Locally, folks are used to paying too much for guns from their pawn shops and gun stores. Exploit that with lower costs. With the online sells, you're reaching anybody in the country who's smart enough to search the net for their deals. The buyers will be more likely to know what their looking for. Put your hot brands and models there.
Oh, and one more thing, stay away from the $150 and below POS guns. They just make you look cheap and desperate. Besides, the MOST you can possibly make on a $100 gun is $100.