Demus had placed the Internet ad and then mislead Marton, who arrived at an apartment complex parking lot in June 2007 with cash, Dallas police said. Marton had traveled from Axtell, near Waco, after finding the car posted on the classified advertising site. Police said George Carter, 24, who conspired with Demus, shot Marton during the robbery attempt.
Carter pleaded guilty to capital murder last year but has not been sentenced.
Efforts to call Demus' attorney for comment were unsuccessful Thursday.
Marton's mother, Mariana Marton, praised the jury's decision.
"It won't bring Christian back, but it was the right decision," she told the Waco Tribune-Herald in Thursday's online edition.
Slideshow archive | ABC13 wireless | Headlines at a glance