ATF wants you to actually have a storefront nowadays. To be perfectly honest gun and ammo business is a royal painand you need something else to eek out a living. People will constantly nickel and dime you to death. Remember you have to compete with Gander ,Cabelas,Bass Pro, Dicks,Sportsmans Warehouse etc etc etc not to mention local well established shops. All have better buying power and can sell at your cost or lower in most cases unless you're fortunate to have deep pockets to make a good deal with the manufactuor direct.
There is a decent profit to be made buying and selling used guns but then you really must know your products and be willing and able to sit on something until you can get your price.