I’d be pissed if they advertised as tyres under 50% worn and they weren’t…
I’d also be more concerned about the mismatched and wrong size bit, have
you checked the sizes?
But then I’d also never buy a car unseen off ebay, and if I knew nothing about cars (not even enough to check for tyre condition and matching sizes) I’d take someone with me who did when I went to pick it up.
That’s not meant as a dig BTW, more of a reminder to learn from this experience whatever happens as there will always be unscrupulous types out there and if you’re not knowledgeable about what you’re buying then you need to protect yourself however you can, either by educating yourself, enlisting the help of others or by paying the premium of buying from a (hopefully reputable) dealer so you have some comeback.
have you contacted the seller about it?