OK, first the actual law:
Definitions first, and these really are important to understand. The definitions are NOT fuzzy. Most of this is in GCA:
(11) The term "dealer" means (A) any person engaged in the business of selling firearms at wholesale or retail,
Important term there, "in the business", also defined:
(21) The term "engaged in the busi- ness" means—(C) as applied to a dealer in fire- arms, as defined in section 921(a)(11)(A), a person who devotes time, attention, and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms, but such term shall not include a person who makes occasional sales, exchanges, or purchases of firearms for the en- hancement of a personal collection or for a hobby, or who sells all or part of his personal collection of firearms;
Read it carefully. To be a dealer you must be "in the business". C&R is clearly not in the business as long as you meet the exceptions in the law. Notice there is no discussion of quantity or reason in the selling part.
The term "with the principal objective of livelihood and profit" is defined and this is especially important because this is where "intent" comes in:
(22) The term "with the principal objective of livelihood and profit" means that the intent underlying the sale or disposition of firearms is pre- dominantly one of obtaining livelihood and pecuniary gain, as opposed to other intents, such as improving or liq- uidating a personal firearms collection:
Note that nowhere in the law are there any discussions of how many guns, how long you have to keep them, or any of the other nonsense that's tossed around regularly. Those things simply do not enter into the law.
It's also important to note that you CAN sell a firearm as a C&R and make a profit on it, that making a profit is not the defining factor of "in the business".
Certainly if you are ever put under the microscope by ATF you need to be able to demonstrate that you are not meeting the definition of in the business, and if you are dealing in C&R to such a quantity where that might happen then absolutely, it's time for an 01FFL.
Keep good records, including money info, and follow the law.