Grand Theft Auto IV is among the games classified as an 18
Children are able to illegally buy violent video games through online auction websites, the UK's Trading Standards Institute has said.
Almost 90% of retailers tested by the association sold under-18s games, such as Manhunt 2, through such outlets.
Traders supplying games to an under-age person in breach of official classifications can face a fine or up to six months in jail.
The institute said online traders had to ensure they complied with laws.
It added that parents and guardians also had "an important role to play in making sure their children are not playing unsuitable video games".
The report came ahead of the institute's conference in Bournemouth.
Welsh Heads of Trading Standards (WHOTS) carried out the investigation. Six local authorities enlisted volunteers aged between 12 and 16, who then tried to buy 18-rated video games online, paying for them with postal orders.
Of the 44 purchases attempted, 38 traders went through with the transactions.
The games, which also included Grand Theft Auto IV, Godfather and Hitman, all carry a British Board of Film Classification (BBFC) rating of 18 because they contain violent scenes.
"This survey shows how easily children can gain access to age-restricted, violent video games," said Lee Jones, acting head of trading standards at Bridgend County Borough Council.
Brandon Cook, the institute's lead officer for age-restricted sales, said that there were several companies who would run age checks on customers for internet traders, which were "very quick and relatively cheap".
Traders selling goods over the internet had "a responsibility to make sure they have methods in place to avoid breaking the law by making under-age sales", he added.
"If traders cannot be sure the person they are selling to is over 18, then they should not be selling."
Game ratings are under increased scrutiny following the decision to give Manhunt 2 an 18 certificate in the UK earlier this year.
The game was approved for release after a nine-month battle between developers Rockstar and the BBFC.
A Private Member's Bill proposing more government oversight of the BBFC was tabled in February but was talked out.