To obtain a refund for an item that didn't match the description, you don't need to make a complaint; you can open a case under eBay's money back guarantee, through the resolution centre: https://resolutioncentre.ebay.co.uk/
If you only wish to complain, the obvious thing to do is leave feedback to alert other buyers. EBay itself is unlikely to investigate complaints by ordinary members; the user agreement makes it clear that responsibility for items sold here rests with the seller.
If the seller is a UK business seller you can report concerns about product safety to trading standards.
If the seller is abroad, realistically there is little that can be done, unless they have a distribution base in the UK. Even then, any meaningful enforcement may be impossible if the seller is in another country. There is for example a well known risk that items bought direct from sellers in the far East will not comply with UK safety standards.
