In the couple of times that I've see this, it was caused by a loose or bad ribbon cable between the lower part of the laptop and the upper ... it can also be a bad LCD. Cold weather, hot weather, dropping it, and just the repeated action of opening and closing the laptop can eventually lead to such behavior.
When you order an item off eBay, it does not come with a warranty unless you separately purchased the warranty, or if the item was purchased new from an authorized reseller. eBay's Money Back Guarantee is intended to ensure that you receive a product in working order, but there is no guarantee that it will work 15 days, 30 days, or a year after purchase (that's the job of a warranty). So, if it worked properly when you received it (and apparently for 30 days more), then the responsibility of eBay and the seller is satisfied.
PS. The replacement of a ribbon cable is not that difficult for a tech person, and the cost is usually under $30.
