The only way to be sure that an Apple product you are about to buy is genuine is to buy it from an Apple or Apple-authorized store. Valid serial numbers can be faked just as easily as devices.
It's possible, of course, that the seller mistakenly described them as 2nd gen, but in that case you might have overpaid.
The fact that Apple's checker says that the serial number is not for Airpods Max 2 would be enough for me to be want a refund, whether or not they are genuine. Even if they are genuine, you'll spend the rest of your ownership wondering and worrying. If it were me I'd raise an INAD with eBay and use the screenshot as evidence to return them as "not as described" and get my money back.