Police are urging people to remain vigilant when using internet selling sites following three thefts in Hemel Hempstead over the past week.
On each occasion, the victims were contacted by a buyer on Gumtree to purchase a mobile phone.
The buyer then arranged to meet the victim in a private place to complete the sale. When the seller handed over the phone, the buyer ran off without paying.
The buyer’s Gumtree profiles were found to have no username details so they could not be traced.
It is currently believed that the incidents are linked.
PCSO Alex Mason, from the Safer Neighbourhood Team, said: “Whilst we would not want to discourage people from using well-known selling sites we would always urge people to exercise caution when doing so, especially when payment has not been made in advance.
“Make sure you look at the buyer’s profile to check they have genuine details. If you arrange to meet someone to sell an item, always meet in a public place, preferably covered by CCTV, and take a friend with you if possible.
“If you have any suspicions about the person you are dealing with, walk away.”