I had a gun salesman at Walmart tell me you had to have a FFL to sell ammunition, and if ATF would enforce that law most of the 22 shortage would end.
The gun salesman lied to you.
Private sale of ammunition is allowed, just like private gun sales.
Firearms FAQs
http://www.atf.gov/content/Firearms/firearms-industry/FAQ-firearms
Q: Is a license required to engage in the business of selling small arms ammunition?
A: No. A license is not required for a dealer in ammunition only, but a manufacturer or an importer of ammunition must be licensed.
[18 U.S.C. 922 (a)(1)(B)]
Private sale of ammunition is allowed, just like private gun sales.
You can make private sales of firearms, but not be in the business of selling firearms without a FFL. That is not true for ammo alone. You could become a high volume dealer of ammunition and it be the only source of your income without a FFL. However, If you did, I suspect you would be “inspected” on a regular basis.