The UK’s consumer organisation Which? found that the illegal weapons were being sold by third-party sellers who came under different names.
UK law - including the Offensive Weapons Act 2019 - prohibits the sale of these weapons in the country and they cannot be owned in a household.
If a person were to be caught with illegal weapons, they could face arrest and a prison sentence.
"It is disturbing that our latest investigation has uncovered illegal weapons being sold on online marketplaces at extraordinarily cheap prices and that these tech firms are also pushing additional dangerous items to people," said Sue Davies, head of consumer protection policy at Which?.
"This raises big question marks over the checks and monitoring being done by these platforms,” she added, urging online marketplaces need to take “more responsibility” and prevent illegal weapons from appearing on their sites.
Searching on Amazon, Which? discovered friction batons - which are illegal to buy and sell in the United Kingdom - were “masquerading as trekking poles” on the site.
According to the group, some of the listings used special characters, such as "bătõň," in the title or only used the word baton in the artwork.
The consumer group said this was presumably to avoid detection by Amazon, while some of the images on the listings indicated that they could be used for self-defence and fighting.
On eBay, the investigation found illegal knuckle dusters, swords and Zombie knives and a 27-inch zombie blade. EBay's terms and conditions state that none of these weapons should be sold on its platform.
Experts said on the website AliExpress, they found a "large number of flick knives, hidden blades, and a raft of ‘self-defence rings’" designed to be worn as knuckle dusters, priced at just £0.49 (€0.58).
On Wish, the consumer group found concealed blades and several belt buckle knives.
Amazon said it would take "appropriate enforcement" against the baton sellers and that it had removed the “products in question”.
“We are disappointed when bad actors evade our systems and we will use these learnings to improve our prevention mechanisms. These products have now been removed from the store,” the company said in a statement.
EBay said it was investigating why the items were not blocked and said it takes the safety of its customers “very seriously”.
“As an international marketplace our policies vary across the world and some of the listings highlighted by Which? are permitted under US policy and sold by US registered sellers,” it said in a statement.
“UK buyers are blocked from purchasing these knives. We are investigating why this block did not take place in this instance”.
Wish said it was "exploring remedial action against any offending merchants".
In a statement, the company said: "While we have a number of systems and methods in place to prevent the appearance of illegal weapons, it appears, on this occasion, our merchants have identified a way to circumvent our processes”.
AliExpress said it had removed the listings and was launching an investigation.