Depends on what you are buying. If it is a recent product you can save a little bit buying used. If you are trying to SLI or Crossfire something you already have, used is sometimes your only option.
Try to stick with sellers that are regional to avoid high shipping costs (and to avoid language specific card issues)
Generally you will want to see a physical marker, such as a piece of paper with the ebay user name in one of the photos. Images of the card plugged in and working is always a plus. If they have the original box they probably intended to resell (Many people do this periodically so they always have the latest)
If they don't state it, ask them their reason for selling. Ask about overclocking as well.