If I buy an Apple Watch on Ebay, am I able to add Apple Care once I get?

Probably not. You can only add AppleCare+ to a device that has been purchased from Apple or an Authorized Seller witching 60 days. So unless the eBay seller has had the Watch for less than 60 days (which is pretty unlikely) you won’t be able to add AppleCare+.

Yes, you will have 60 days from date of purchase. Check out this link.....watch