A transaction from the app LetGo went terribly wrong for one teenager in Melrose who thought he was buying an iPhone.

News 12 is told the 19-year-old victim met two men on Dec. 30 along 149 Street and Cortlandt Avenue.

The victim tells investigators he showed up around 5:20 p.m. and was given the iPhone, but right after, the two men pulled out a knife and demanded his money while taking the cellphone back.

News 12 is told the victim wasn’t hurt, but was robbed of $360.

Police are warning people to be careful when meeting sellers online. They say they've had other incidents involving the LetGo app.