Not sure I would trust the warranty being honored if the only receipt is a paypal payment statement from an eBay auction. Staples was selling this earlier this week (now OOS) for $159, which was a cheaper deal when combined with coupons.
You don't need a receipt for warranty service on a hard drive. They don't care when you bought it or who you bought it from. The warranty in based on the drives serial number and starts the day the drive leaves the factory (plus a grace period).
The drive serial number is not on your receipt, only the model number. They don't want me to buy this drive today and then another of the exact same model 9 months from now. When this first drive fails, if it is out of warranty, I could just use the receipt from the second drive to claim it is still under warranty. Every serial number has it's own warranty. That is why you can pop the drive out of the external enclosure and get warranty service on the bare drive using the serial number on the drive itself. If they kept track of which drive was in which external enclosure the drive and external enclosure would have the same serial number.