The university's athletic department said Willis, a junior, violated its student-athlete code and NCAA rules regarding his athletic scholarship by selling the books. In addition to the suspension, Wills must pay back an unspecified amount of money and perform community service.
Willis, who is averaging 6.8 points and 5.0 rebounds for the Catamounts, has already sat out three of the past eight games. He won't be eligible to play until Jan. 18, when Vermont hosts UMass Lowell.
In a statement, the Indianapolis native apologized and said he realizes his actions were wrong.
A spokesman for the men's basketball team called the incident a disciplinary issue and said he couldn't comment on why Wills wasn't allowed to sell the books.