can I buy AppleCare for a MacBook purchased on ebay?

No.

Only for brand new from an official Apple Reseller -OR- Apple Certified Refurbished (which are indistinguishable from new) from the Apple Refurbished Store.

No Apple warranties available otherwise. Bid Appropriately.

No... but a used Mac with an existing AppleCare Agreement can be sold. AppleCare stays with the product and it's an attractive selling feature.