A 17-year-old robber’s arm was severed in Brooklyn after he held up a man at gunpoint for a pair of pricey sneakers, police sources said.

Through the app Wallapop, gunman Zachary Sam and seller Philippe Pierre, 39, had arranged to meet up at 1 p.m. Friday in Canarsie, sources said.

But when Pierre showed up, asking $490 for a pair of Air Jordan 8 Retro sneakers, the teen crook pulled out a gun and demanded the kicks for free inside Pierre’s gray Honda Pilot, police sources said.

Pierre stepped on the gas pedal, but Sam, who lives nearby, jumped out of his car at the intersection of East 86th Street and Avenue M, sources said.

Instead of speeding off, Pierre turned his car around and drove after Sam, crashing into him in front of a fence.

The alleged robber’s arm was ripped off when he was pinned against the fence.

“I saw a kid under a car,” said Alex Saint Fleur, a bus driver who lives across the street.

“The guy ran him over. He got out, the driver said, ‘He’s trying to rob me. He’s trying to rob me.’”

“I saw the gun on the floor,” Fleur added. “The arm was on the floor near the gun.”

Sam got out from under the car, he said, and ran inside a city bus.

“Everyone is screaming, ‘Come back, come back, your arm. You’re bleeding too much,’” Fleur said.

Sam then got out of the bus and started running down Avenue M, he said, before finally collapsing in the street.

The teen was taken to Brookdale Hospital, where he remains in serious but stable condition, law enforcement sources said.

On Friday afternoon, the bus was pulled over on 80th Street, blood spattered on its front entrance.

“I was walking up the sidewalk and I heard a boom behind me,” said a postal worker who declined to give her name. “I just kept it moving. I saw somebody run on the bus with a missing arm.”

Pierre was arrested and charged with attempted murder, sources said.

Cops charged Sam with robbery and criminal possession of a weapon.

With reporting by Natalie Musumeci