Where did you buy your ammo? Was it Powder Smith in La Plata, or the Walmart?
On the MDshooters page, there was a thread about a seller (Dick's I think but not sure) who got busted for illegal sales and agreed to turn over buyer info. Most stores don't. I've never been to a store that wouldn't accept cash and no ID for ammo (for example, Bass Pro Shop in Hanover MD, the Armory in Annapolis, the various stores in Severn MD). If you're in La Plata, think about driving down to
Springers Guns & Tackle - 13035 Kings Hwy, King George, VA.
I think it IS a good idea for the government to flag large strange purchases of ammo. It sounds like their 'system' is overly sensitive. As an uniformed opinion, I'd say $2000+ of ammo per month should trigger police interest - check to see if the guy is buying for a club, has a job, has prior drug convictions, etc (not an after hours visit).