A Humberside Police spokesman said the couple in their 70s were approached at their home on Derrythorpe by a man offering them 10 small trays of what he claimed to be Grimsby fish.
The couple did not agree to a price prior to purchase, and made a payment with a mobile card machine.
The man had left by the time the couple realised he had processed a payment for £376, but luckily, they were able to stop the payment before any money went out of their account.
Upon inspection, officers found the fish was actually bought from a local supermarket.
Whilst still in date, it is believed it had not been stored at the correct temperature.
PC Brendan Franey from Humberside Police said: “Our advice is don't do business on the doorstep. Don't feel you have to be polite and listen to a sales pitch if you’re suspicious.”
"We do hear some really quite horrible stories about people who have lost hundreds of pounds in these scams or more. It's not unusual for these individuals to charge up to £400.
"There are all sorts of issues around the product. You might think you're getting haddock for example but you’re actually getting something much cheaper. On this occasion there was a packet of kippers, ordinarily a very cheap fish”
"Don't assume you are getting a bargain.”
Local trading standards officers have been made aware. If you have any information please call 101 quoting log 216 of 07/02/18.