The talent that Mark had was the ability to hot wire anything with wheels. He was a car thief and as a direct result was on probation. This is pointed out further when he got into more trouble after having stolen the Principal's car in order to go meet with his parole officer because he had no other way to get there. Some how though, Mark always inevitably finds himself in someones good graces and doesn't really get into as much trouble as he should for the actions he does. That is, until the end...

What 'talent' did Mark have, and what happened to him because of it?

Mark had a gift for getting away with things. This trait, coupled with a skewed obliviousness of right and wrong, allowed him to habitually engage in activities like stealing, hustling, and hot-wiring cars without getting caught, or if he was caught, to get off lightly. His transgressions progressed to more serious offenses, until finally he became a drug dealer, was turned in by his best friend, and was sentenced to the reformatory and ultimately the state penitentiary, where he became a hardened, dangerous character who couldn't get away with things anymore.