Most gun wholesalers will not give you can account unless you have an ffl and a state tax # minimum.
Also trying to sell various gun parts online has a lot of state restrictions and competition from other already established online sites.
The only way you can even think of competing would be to deal directly with the parts manufactures, when ever you deal with the various USA distributors the mark up will be so high that you wll never be able to sell your products online.
Then comes the next problem, in order to get the deep discounts you need from the manufacturers you wll have to place big orders usually $50K - $100 +
In todays online market place, don't even think of selling locally retail. Most local retailers go belly up with in there first year, as they can not compete with online sales.
The example rhino gives of his bussiness, is a good example of niche, bussiness where insider information and contacts can pay off well, but you can't use this bussiness example to every situation.
You have to look closely at the current market, customer type or demographics, existing competition and pricing, market forecast.
One thing you should do is get a few used text books from a local college etc on bussiness 101 and management 101, or even just barnes and noble etc. Get the basics down before you take any deep plunge into loans or inventory purchases.
In doing a bussiness forecast or plan you really need to take into account things like rent, office space, mandatory taxes, inventory, expected profit etc. health care costs alone are about $6000 a year for 1 person and about $10K for a 2 person operation
One of the first issues will be the high cost of overhead, in just rent and utilities etc. you will often find that a lot of small bussiness barely make enough to break even and that it would be wiser to just work a 9-5 job then continue making almost nothing at their own bussiness.
That is why I mentioned to focus on another carrer choice like government service, law enforcement. You really can't beat the GIGS that most cops have $75K - $100K per year with OT and retirement after 20 years at 50% of their last 3-4 yrs. pay, and 80 % after 30 years, + free health care for life.
I know a lot of cops that have retired after 20 yrs. getting $40-$50K per year and full health care and then start their own bussiness or second career.