This should be easy to clear up.
Apple serializes products and the warranty starts when the device is first sold. Since you are in possession of a device, register the device to your Apple ID with Apple and then ask Apple when was the original sold date.
If the listing says an apple agent warrantee is in place, I would take the device to any Apple agent you choose and get in writing that they’ve denied you warranty service.
Now you have a paper trail to take to eBay if the listing was falsified. If the listing wasn’t falsified then you bought something without the knowledge the seller had. Unfortunately this is a “feature” of many “markets” when the seller has more information than the buyer.
Since eBay is the market here, hopefully the market will compensate you if in fact the seller took illegal advantage of the marketplace rules. Or they will educate you in how their market operates and then you decide if you want to pay for out of warranty repairs.
If the seller observed the rules, there is a saying for this - https://en.wikipedia.org/wiki/Caveat_emptor