Gloucestershire police are also warning people buying expensive items from unknown people that they risk being accused of buying stolen property.
A spokeswoman said people were handing over cash for cut-price goods and being given low-value items or rubbish.
The warning follows a number of incidents in Gloucester and Cheltenham.
"People buying electrical goods should also ask to see them working before parting with their cash"
The spokeswoman added the people involved were skilled at deception and switching the items even after the victim had examined the goods.
She said: "In one incident at a car boot sale in Cheltenham the victim handed over £600 in cash believing he was buying two laptops.
"When he examined the contents later he discovered he had ended up with a bag containing bricks and two bottles of water.
'Lemonade bottles'
"In a further case, a man believing he was purchasing a laptop, a satellite navigation unit and a camcorder for £530, found himself left with three bottles of lemonade and some cardboard."
Martin Bruton, from Gloucestershire Trading Standards, said people buying high-valued goods from unknown sellers were potentially risking their cash.
"Our advice is that if the deal sounds too good to be true then it probably is.
"People buying electrical goods should also ask to see them working before parting with their cash."
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