A couple thinking they'd bought a laptop computer opened the box to find bottles of water.

Other victims have found potatoes or bottles of lemonade in their boxes.

Police said tricksters were getting away with up to £600 each time and were mocking the victims by filling boxes with worthless items.

The latest victims were a couple who bought from a man at a car boot sale at Brighton Station at the weekend.

He was one of two men, both white and about 30, who approached people and showed them a laptop. Victims are going to nearby cash point machines and withdrawing the money to pay.

The station transaction was completed in the back of a green or blue old-style car which had its engine running.

The couple were handed a sealed box and got out.

They opened the box straight away but the car sped away before they could challenge the villains.

Similar scams have been operating from the backs of vans at Brighton Marina and at Eastbourne railway station.

Sussex Police have 30 incidents listed and police fear there may be many more.

PC John Dickinson, investigating the weekend incident, estimated that for every case they record there are probably five others.

He said: "People are often too embarrassed or ashamed to report it. But we would urge them to come forward. We need as much information about these conmen as possible.

"The couple at the weekend felt stupid and very upset but we are very sympathetic. We would urge everyone to be aware and to be on their guard."

British Transport Police are launching a poster campaign at stations warning people and they urged anyone with information about the conmen to come forward.

Call PC Dickinson on 01273 328545.