On Thursday afternoon, staff at a business in Belleek were approached by a man saying he had "left over line painting material" and offering to lay out a car park. The staff declined.
Last week a similar incident took place at another business in Fermanagh.
A police spokesman said: "The 'left over material' ruse is one that is often used by cold callers to businesses, and homes. At other times it may well be 'a load of tarmac left over and we would only have to dump it' type of approach.
"At the very least, the material is likely to be sub-standard and the work shoddy. Those involved may also try to charge an exorbitant amount if they think their customer is naive about pricing.
"If you want work done about your home, approach local, established companies or individuals that you can talk to again if anything goes wrong."