i wouldnt think itd be worth it measuring risk vs reward. call me biased because i got burned, but youre better off with actual gaurantees at actual stores. so... at least make sure its a reputable seller or storefront.
Also, check Newegg.com and other places to make sure it is actually a good deal. Some things on eBay are cheaper than Best Buy or Wal-Mart, but NOT cheaper than Newegg.com or other big online retailers.