Police are hunting a thief who robbed a man of his $20,000 Rolex watch during a meet up in Melbourne arranged through the online marketplace Gumtree.

The seller arranged a meeting at a coffee shop in the Doncaster shopping centre, in the city's east last month, after advertising the Rolex Cosmograph Daytona for sale.

He discussed the price of the watch with the thief, who then asked to try it on, police said.

The thief then fled with the watch on his wrist.

The victim chased after the man but stopped when the thief pulled out a weapon, believed to be a screwdriver, from his pocket.

The thief was described as being about 180 centimetres tall, between 20 to 30 years old, with an athletic build.

He used the name Johnny during the transaction.

The watch had a brown Arabic dial and an 18 carat rose gold folding buckle.

Detective Senior Constable Leanne Trusler said people buying and selling items using Gumtree and similar sites needed to be cautious.

Police want to speak to this man over the theft of the expensive watch. ( Supplied: Victoria Police )

"I'd make some follow-up calls to make sure you're meeting someone who they say they are," she said.

"But selling a piece of jewellery or a watch at that price, I'd be doing it via other means.

"Lucky for [the couple] nothing has happened to them, they've had an awful shock and they're out of pocket but they've not been injured."

The couple are expecting a child and had wanted to put the money away for the future.

Detective Senior Constable Trusler said it was wise to meet in a public place and not at home.

"Anything could happen. You're never sure of what people are up to."