Why Groupon Is No Ebay

Groupon and the like don't promote customer loyalty. They promote disloyalty. Why go back to the same restaurant when I can try another restaurant for 50% off?

Each vendor would have to evaluate with ROI with Groupon and no one really know how good or bad it is because it is likely to be kept secret. No one likes to be shown to be a sucker if it is bad, and no one will share if it is great.