For an example of what can be done with such purchased data, one project at a previous employer was:
- identifying all cell phone #s which would regularly appear w/in a certain radius of any State Police Barracks
- disambiguating that from people who lived/worked nearby and/or who met certain criteria
- determining the income and certain other criteria of the remaining numbers
- identifying the home address of the remaining cell #s which met the final criteria and mailing a franchise offer to those cell #s with the assumption that it would be targeting State Police Troopers
In fairness, I mean, if the people collecting the data sell it on the open market, then you can't realistically expect it to be private.
The only solution in that case is to make it illegal to sell the data. And that's never gonna happen in the US.