The point is, what the government is proposing here is insane. When they talk about tracking purchases, there is simply no means to control and gather that data - and control would be a key part of the equation. You have the issue of cash, and you have the issue of the fact that even credit card data only captures date, store and amount. Was that $400 purchase at Wal-Mart a gun? Or a Stairmaster?Was that expenditure at a farm supply ammonium nitrate? Or seed corn and a part for the tractor?
So the feds would have to mandate that all item purchase info be captured. And require that folks paying cash provide ID and the ID be captured. And that all point of sale systems be modified to capture this information and transmit it to the government in a standardized format. And the data be loaded, matched and merged in something resembling realtime, when the IRS can't even implement systems where 99 percent of the data received has a linking key.
So either DARPA is lying to us that existing data will suffice (experience with direct marketing shows it will not), or they are lying in that they intend to devise ways to gather more data, or they don't have a clue as to what this project will require. It's that simple.