Is there a limit to the number of guns per month (year?) that a person can sell before the authorities consider you to be a "dealer" and require more oversight?
AFAIK only California has such a law.
Penal Code 26500
(a) No person shall sell, lease, or transfer firearms unless the person has been issued a license pursuant to Article 1 (commencing with Section 26700) and Article 2 (commencing with Section 26800) of Chapter 2.
(b) Any person violating this article is guilty of a misdemeanor.
Penal Code 26520
(a) Section 26500 does not apply to the infrequent sale, lease, or transfer of firearms.
(b) As used in this section, "infrequent" has the meaning provided in Section 16730.
Penal Code 16730
(a) As used in Section 31815 and in Division 6 (commencing with Section 26500) of Title 4, "infrequent" means:
(1) For handguns, less than six transactions per calendar year.
(2) For firearms other than handguns, occasional and without regularity.
(b) As used in Section 27900, the term "infrequent" shall not be construed to prohibit different local chapters of the same nonprofit corporation from conducting auctions or similar events, provided the individual local chapter conducts the auctions or similar events infrequently. It is the intent of the Legislature that different local chapters, representing different localities, be entitled to invoke the exemption created by Section 27900, notwithstanding the frequency with which other chapters of the same nonprofit corporation
may conduct auctions or similar events.
(c) As used in this section, "transaction" means a single sale, lease, or transfer of any number of handguns.