I think this is a case of screwed up Overture #'s
If you check using DigitalPoint.com's tool you'll find WordTracker reporting 44 searches/day for "buy book online" and Overture reporting 7587.9 searches/day.
I suspect that WordTracker is more accurate in this case.
Consider that Overture reports only 14.3/day searches for "buy used book online" and WordTracker says 4/day.
Using only one tool, like Overture, as justification for a domain name purchase is just not good enough.