The suspect, Michael Gordon, was convicted of first-degree murder and armed criminal action.
Prosecutors said Gordon, 27, shot and killed Taylor Clark at a Hazelwood truck driver training school where Gordon worked.
Clark, a St. Jacob, Illinois native, was trying to sell his car on Craigslist and Gordon responded to the ad. Police said Clark met Gordon at the business where Gordon was supposedly going to test drive the car during his lunch break. Instead, investigators said Gordon shot Clark in the head and killing him. Gordon then tried to hide Clark's body and returned to work.
Clark's body and his car were found the next day.