The owners of 136 vehicles for sale on the sides of roads in Caerphilly County Borough have been warned by Trading Standards.
Since January this year, it has been illegal to sell any goods on council land without prior permission.
This includes cars which can cause a danger to other road users by blocking sight lines and causing potential purchasers to park dangerously to have a look.
As well as this, Trading Standards have also uncovered a number of traders pretending to be private sellers to avoid their legal obligations to customers.
Cllr Rob Gough, cabinet member for public protection said: “Our Trading Standards and Highways officers continue to work hard to tackle this issue.
“In addition, our officers are now able to share information with Trading Standards officers right across the country about roadside sellers, to help catch the unscrupulous rogue traders who set out to deliberately mislead innocent purchasers.”
Over four weekends in October, 21 vehicles were targeted by Trading Standards and the owners called out to speak to officers. The majority were private individuals who were unaware of the law, and advice was given.
A number of traders have been unearthed however, and Trading Standards are now investigating.