A warranty can cover a product bought on eBay as long as it comes with an existing warranty from the seller and is new or refurbished. However, you cannot buy a warranty on used goods.
Having an existing warranty to support a product ensures the product is in good working order when sold.
You can buy an additional service plan for a product sold on eBay if it meets certain conditions:
Warranties sold on eBay from Geek Squad (Best Buy) or Square Trade are often more expensive or limited in type of coverage you can buy.