I've just found an awful listing on eBay, a fish (mickey mouse platy) in a keychain. It mentions a "production line" so this isn't something where they're picking up the fish that have died naturally, these are fish that are being bred then killed specifically for this purpose in Hong Kong (apparently). Is this legal and should this be allowed on eBay?

That's awful Not sure what eBay policy would be. They prohibit fossils, artefacts etc and don't allow stuffed animals (where it breaks defra rule)so may not tolerate this, though it might depend on proving the fish are killed specifically for the purpose. Might be worth contacting them with the listing number. I suppose there may be general prohibition in importing this kind of thing into the UK so may be worth checking Customs site regarding that. If they're not legal for import then eBay should be able to ban them at least on the UK site.

They are from Hong Kong, so not sure what rules they have for that.

I just nipped over to eBay and took a look. If its the same seller you found electrogear, then they have an ebay shop as well. Nauseating is the only way I can describe it. Unfortunately the seller is getting around it by selling the stuff as 'specimens' for scientific purposes. I've still reported them anyway.