It happened at around 11.45am today (21/11) in Henley Road, Caversham outside the parade of shops.
The victim, aged in his thirties, was waiting to meet a man he had agreed to sell his watch to which had been organised through a buy and sell website.
He was approached by the offender who took the watch and then ran away without paying.
The victim chased after the offender who then dropped his phone. He threatened the victim with a large knife, picked up his phone and ran away.
The offender is described as a black man, aged between his late teens and early to mid twenties, approximately 6ft 3ins tall and with a slim build. He was wearing a black hooded top with a full length zip that was done up and with the hood up. There was red around the neck of the hooded top. He was also wearing black tracksuit bottoms, a covering over the bottom half of his face that may have been a scarf and was carrying a black rucksack.
Investigating officer, PC Thomas Wheatman, based at Reading police station, said: “I am appealing for any witnesses to this incident, or anyone who believes they may have information about what happened to please come forward.
“The watch that was stolen was a Rolex Datejust 41 with a stainless steel bracelet strap and a white dial face.
“Anyone with information is asked to contact police by calling 101, or making a report online, quoting reference 43200387465.
“Alternatively if you wish to remain anonymous, you can contact the independent charity Crimestoppers on 0800 555 111.”