Comment by dasfasf

Just "bought" implies that the sale of the books is compassed in the past. It sounds strange because no explicit compass is mentioned and the reader suspects they are still for sale. I don't see anything wrong with "at least" though. Note that "over" changes the meaning (>= vs. >) and at least some prescriptivists obelize using "over" like that (I believe the AP Stylebook used to).