Amazon Marketplace and eBay are still being used by opportunist sellers to cash in on people in need, Which? has found. The consumer group said it had found overpricing of cleaning products, thermometers, baby formula and tampons.
In several cases prices set 1000% higher than shoppers would normally expect to pay and in some instances the price inflation was even higher.
Which? said online marketplaces should be bringing in stricter controls to identify and prevent sellers charging unjustifiable high prices for essentials - and policies must be communicated clearly and directly with sellers.
But both eBay and Amazon said they are taking tough action against "price gouging" - which happens when a seller dramatically inflates the price of