A man had thousands of pounds stolen from him before being threatened with a handgun during an incident in Greenleys.
Three men aged 60, 35 and 28 have been arrested on suspicion of robbery and possession of a firearm after the robbery in Pinders Croft last Saturday around 7pm.
The victim, 31, arrived in a car park by taxi responding to an advert for a blue BMW 3 Series.
When he arrived to meet the seller, he saw three men standing against a wall who then approached him and grabbed his jacket, causing his money to fall on the floor, which was stolen.
As he ran away, he was threatened with a black handgun, as was a witness who tried to help him.
Detective Sergeant Luke Grove, of Milton Keynes CID, said: “I would like to speak to anyone who saw this incident or who saw the men in the car park area around the time it happened.
“I want to advise people to take sensible safety precautions when responding to online adverts over Gumtree or any other similar websites. If an advert appears too good to be true, then it probably is.
“I would advise people not to carry large sums of money when meeting proposed sellers on the first occasion; always meet in a well-lit public place and always get full contact details for any potential buyer or sellers before you meet them.”
Any witnesses can call police on 101 or Crimestoppers, anonymously, on 0800 555 111.
The 65 and 35-year-olds have been released on bail until June 16 and the 28-year-old has been released on bail until July 1.