Depending on what the "pre-determined amount" is, the police are going to get very tired of the notifications very quickly. It's not as if they can actually investigate anything based on the reports, since the mere fact that you bought ammo doesn't give them a basis for a search warrant. At most, the police could call the buyer or stop by to ask if he has any nefarious plans for the ammo, but the buyer doesn't have to take their call or answer the door.
you say this and yet they do it routinely in Australia. not long ago a member here posted that he filled out all their mandatory paperwork and forms and licenses and so forth and bought a 3006 pump action rifle and 250 rounds of ammo, a few days later 2 men arrived at his house with a search warrant and proceeded to search his place for anything 'suspicious'. naturally all he had was a single pump action rifle and a few boxes of ammo so they let him off with a friendly warning to watch how much ammo he buys as it sets off red flags and they are required to investigate.
if those laws pass, law enforcement will be required to investigate and I'm sure search warrants will be made mandatory for such a deal so you have no choice but to submit to being searched. regardless of how sick of it the cops may get, they will still be required to investigate and they will do it whether they like it or not.