The council said a lack of a receipt made tracing the traders difficult
A warning about doorstep fish salesmen talking people into buying up to £1,000 of frozen fish has been issued by Lancashire County Council.
Some customers thought they were buying a few fish pieces but were landed with a van load, much of it close to its use-by date.
A council spokesman advised residents not to deal with unknown cold-callers.
"You are entering into uncharted waters by buying on the doorstep," Councillor Albert Atkinson said.
Jim Potts, chief trading standards officer added: "We have some concerns that the fish may not be accurately described or is the correct weight, or that doorstep cooling-off rights are being given.
"When buying fish this way, you have no way of knowing whether it is the correct weight, the correct type of fish or even that it is safe to eat, due to how it may have been stored."
He said that a lack of a receipt made it difficult to trace the traders.
Councillor Albert Atkinson, from the council, added: "The best place to buy fish is from a reputable fish monger on the high street or local market, or from the supermarket.
"Several elderly consumers in Lancashire have been misled into thinking they are buying a few pounds worth of fish, only to be shocked to get a whopping bill and more fish than freezers can take."