There is some arbitrary number of guns you can sell in a year before the BATF decides that you are dealing guns without a license- noone seems to know this number.

I know of a few FFL holders who deal out of their houses, they do mail order business and don't stock firearms. This can be done so long as your local ordinances do not prohibit running a business out of your home or prohibit selling guns in your community.

If you have a FFL, you must have a business of selling guns, getting an ffl to buy guns for yourself at a discount price is illegal.