Simple answer. Have a pocket Constitution and ignore it!
I posted on another thread an incident in the Phoenix area where three private citizens were sent letters from the BATF stating that they were now on their “watch list” for buying excessive ammunition. My friend who knows these people has a copy of the letter, and I’m hoping to get a copy for myself. According to her, these individuals also received phone calls from BATF agents. Scary stuff. As to how they knew about the purchases, I don’t have an answer for that. However, at the last gun show in April, my husband was required to show his driver’s license when he purchased some ammo. He used a credit card but said the guys paying cash were also told to show photo ID.