........my buyer purchased those AR lowers with the intent to flip them for profit. ATF sent the letter to make him aware that selling them would require an FFL. When he purchased those lowers he became "engaged in the business".
A small point, he was not "engaged in the business" when he bought them. Hence the warning ATF letter. He would be "engaged in the business" when he sells them.
You can buy any amount for your own personal use. Even 100 AR lowers. Yes, today you will be on some watch list if you do that, but you can do it, and they can't do anything about it, unless/until you go to sell them.
Then it comes down to how convincing an argument can be made. Say I have 100guns, acquired over decades, and decide for what ever reason I need money more than guns, I could sell them all, either as one lot, or having a table at 18 different gun shows over 6 months, and its doubtful even the most zealous prosecutor would look twice.
Buying 100 guns (or just stripped receivers) at once? Even if you sit on them for a decade, they are going to look closely to see if you are technically engaged in the business.
Another case, I knew a guy, Big into Contenders. He was a serious hobbyist. Had a table at all the shows, ran the circuit in about 3 states. Bought some things, traded some things sold some things. Did that for several years. Ran into the ATF (eventually) because he didn't have an FFL. After him being gone off the circuit for several months, he came back. Although it took some time, he was officially ruled a "hobbyist" and not "engaged in the business".
because the law is so vague, the details really matter, and each case is judged by the specifics.
I knew of another guy, who was always "selling from his personal collection". However, his "personal collection" always included a number of new in the box Jennings, Brycos, and other cheap handguns. No matter how many he sold, his "personal collection" was never depleted, show after show. ATF ruled he was engaged in the business, and he went away. Don't know if he became a long term guest in govt housing or not, but haven't seen him at any shows in years now....