On an individual basis!
NOT tracing MILLIONS of buyers!
All the detail is out there for millions of buyers. All they have to do is begin consolidating and focusing in on various buying patterns. That’s exactly what Data Mining is all about. It may take months but they can find everyone who bought .223 ammo.
Another example would be Facebook. They now can scan pictures loaded into a facebook, and with facial recognition, link each person in the picture into a network of friends and known associates. If they have that kind of computing power at their disposal a parsing simple invoice over email is piece of cake.
NSA now stores all emails in a monster database. They can easily triangulate on emails from Midway USA or Cheaper than dirt as a starting point.