The tooth-whitening kits contained almost 60 times the legal limit of hydrogen peroxide, the bleaching agent in tooth-whitening.
The kits could potentially lead to mouth blisters and sensitive gums to users.
Uxbridge Magistrates’ Court has issued a court order for the 12,000 seized kits to be destroyed.
The council reported that the kits belonged to Piotr Kaszubski, a Polish national who has since fled the country.
A spokesman told the Mail Online: ‘Trading standards officers uncovered evidence that the goods were being supplied online to consumers with inadequate checks being made.
‘Mr Kaszubski has returned to Poland and stated that he was merely an agent for an American company, so it was decided to seek forfeiture of the products worth in excess of £500,000.’
The products seized were found to contain 5.9% hydrogen peroxide, meaning they could only legally be issued by dental practitioners.
European law states that tooth-whitening products are only safe to be sold directly to consumers if they contain a maximum of 0.1% hydrogen peroxide.