I suspect eBay consider tyres to be parts of a motorcycle. Probably they expect people who are after tyres to include the term in their searches? Novel idea, I know, but maybe worth a try.
Fwiw - unless it was a special purpose, hard to get tyre I would be buying tyres from a real shop. Where I get a real warranty and don't have to pay exorbitant freight costs. But that's just me.
