That would be because eBay haven't allowed payments except through Managed Payments for several years. You can pay with Paypal (currently, it's due to not become an option soon, if not already).

The seller is attempting to bypass eBay.

You could have issues. eBay has no record of the purchase, and the MBG won't apply. If you don't get your item or if it is not as described you will have to go through Paypal for any redress.

In future, NEVER pay on a seller invoice unless it is through eBay.