The suggestion is that the fish is poor quality, over-priced and not as described.
The council is urging people to get in touch if they have had similar experiences.
Julie Dean, Fair Trading Standards, said: “If possible we need more information on the areas the sellers visited and descriptions of the men and the van. Ideally we need people to come forward who have bought from these men.
“We have received reports of rogue traders operating in Broughton, Burton, Belton and Scunthorpe, and it’s not just fish but people selling beds, offering to tarmac drives and chop down trees.
“If you have been the victim of a rogue trader please contact us on 01724 297659 or if you want more advice contact us on 03454 04 05 06 or visit www.adviceguide.org.uk.
There have been reports of rogue traders operating in the following areas:
Broughton - Fish sellers going door to door selling poor quality, misdescribed and over-priced fish. No paperwork provided.
Burton - A mobility company going door to door selling very expensive beds and chairs using high pressure sales techniques.
Belton - A trader offering to tarmac drives with spare tarmac. The price was agreed per metre but after completion the trader demanded more money by saying the price was per foot which significantly increased the cost. The trader provided false business details.
Scunthorpe - A trader cold called and told the elderly resident that the tree in the garden was unsafe. They charged £250 but only trimmed the tree.