Gwent Police says it has received reports of men cold calling in the Monmouthshire area.
So far the Monmouth, Osbaston, Usk and Goytre have been affected.
The males state they are from Wolverhampton, Scunthorpe and Middlesborough.
They then proceed to try and sell their services by showing a laminated piece of paper to homeowners to prove their legitimacy.
Anyone with information is asked to call police on 101.