The stickers ask owners to get in touch with the council
Traders in Darlington who sell their cars illegally by the side of the road are being warned they risk fines.
The council has received a number of complaints about vehicles parked in the North Road area and labelled as if they were private sales.
Residents have said they are unsightly and cause a nuisance when people come to view them.
Patrolling officers will monitor such vehicles and put warning stickers on the windscreen of any suspicious ones.
These remind owners under the Cleaner Neighbourhood Act it is an offence to sell more than two vehicles within 50m (164ft) of each other on a public highway.
A phone number urges them to get in touch with the council, and if they do not further enquiries will be made with the DVLA about the ownership, tax and insurance of the cars.
If found to be flouting the law, owners face a £100 fine.
Bill Dixon, Darlington Council's cabinet member for neighbourhood services and community safety, said: "This new campaign will ensure we continue to clamp down on these rogue traders and that they are aware that we are watching them.
"These cars are an eyesore, cause problems for residents and could be potentially dangerous."