Motorists are being warned not to buy cheap jewellery from Eastern European salesmen operating around the Hartshead Moor service area, on the M62.
Police said drivers had been targeted by men and women trying to sell suspected fake goods.
The Lower Calder Valley neighbourhood policing team said it was advisable not to buy anything because the jewellery was unlikely to be genuine.
Anyone who is approached is asked to contact Welcome Break staff or the Police Community Support Officer who operates at the services.
The warning comes just months after a crackdown was launched in Bradford to prevent illegal Eastern European street sellers selling fool's gold to city centre shoppers.
In April last year, the campaign resulted in the county's first on-the-spot fines being issued to a group of seven Romanians.
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