An investigation led by the research group found a “consistent overpricing” of household items, including cleaning products, thermometers, baby formula and tampons.
The CMA had previously warned traders to not exploit panic-buying amid the coronavirus outbreak or take advantage of people through price gouging.
Despite this, Which discovered “multiple examples” of essentials items being sold for inflated prices on eBay and Amazon.
In a statement, the group said: “We want marketplaces to take action on those looking to profit from the Covid-19 pandemic – to stop shoppers from being duped into buying essential goods at a significant markup, and removing the incentive for sellers to bulk buy in the first place by removing the offending listings.”
In particular, it uncovered a “dramatic” overpricing of household items that are in high demand in supermarkets, and “hundreds” of active listings with a mark up of up to 1,000%.
It found that 250ml bottles of Carex, which often retail for £1, were seen priced at Amazon and eBay for more than £10 each, with one eBay listing showing the item priced at £100. Another at Amazon was available from a third-party seller for £26.41.
Bleach products, baby formula and thermometers on both sites also had significant mark ups, with one Braun thermometer (rrp £50) selling for £300 on Ebay.
Which concluded there was a failure of ‘filters’ and other checks put in place by eBay and Amazon to “adequately protect consumers”.
Sue Davies, head of consumer protection, Which?, said: “Online marketplaces have taken some action against coronavirus price gouging, but our investigation shows unscrupulous sellers are still cashing in on people’s fears by selling essential items at extortionate prices on eBay and Amazon.
“These companies must make good on their pledges to stamp out coronavirus profiteering and if they fall short the CMA must be ready to take strong enforcement action.”
She said that the government should consider how it will work with the retail sector as a whole to keep the price of essential items “reasonable” as the fallout from the coronavirus outbreak continues.