Michael Henry will spend 20 to 66 years in prison.
The sale was described as a "straw purchase." Straw purchasers are people with no rap sheet who are able and willing to buy guns for convicted felons, who are barred from making that transaction.
Henry sold a gun to Andrew Thomas, who then used it to kill Plymouth Township Police Officer Brad Fox in September, 2012.
Fox was shot while looking for Thomas, who drove down the wrong lane past an accident scene in a stolen SUV. Investigators say Thomas ambushed Fox, firing four shots from a hidden elevated position along the Schuylkill River Trail.
Thomas killed himself after shooting Fox.
Earlier this year, Gov. Tom Corbett signed the Brad Fox law, which included tougher penalties for straw purchasers.