With the prevalance of online dealing and auctions, I dont see why you couldnt do one for an online only store.
"In the business of selling firearms" is very vague, mainly that you are repetitively buying and selling firearms, there are no set number you must sell, its an intent to do business, and you need the license to do so.
I think trolling gunshops for deals to sell on Gunbroker is a legitimate business anymore, hell, Ive found some deals both in person, and online, besides, offer to do transfers for online purchases as well, every auction site has a list, and its one more avenue of business.
If they give you shit, point to Amazon.com, and its total seperation from the brick and mortar business model.
The BATFE may make it difficult for you, but if you pass the background check, meet local zoning laws if there are any, and have a good gun safe, I dont see why they could deny you, if they do, fight the decision, they are denying you a livelyhood.
Now, you will find discrimination from some wholesalers, Accusport comes to mind, they refused to sell to me years ago, when I didnt have a seperate business address, and they have lost THOUSANDS in sales since, then, I've never forgotten, and I will NEVER deal with them, they are not the friend of the small dealer.
One thing is this though, if you think that you are going to do this, and buy 3 guns a year for yourself, and your books show you've never made transfers to someone else, its a good case to deny a renewal, you must make an effort to be a gun dealer, if its to save 20 bucks three times a year, dont bother, its not worth the pain in the ass.