Much of the world is middlemen, and it is often a fairly profitable place to be for companies. (A large multinational advertising firm with 35% margins that employs quite a few PhDs comes to mind.)
They recently introduced group buy as well. I'm quite certain Groupon is aware of this type of competition. If Google is even getting dominated in the Chinese market by Baidu and the government, I doubt Groupon is going to accomplish much success in China. It's too young and too ambitious, and I think throwing money into the Chinese market is like falling into an abyss. That's what I firmly believe because I know the Chinese market quite well.
Prove me wrong Groupon =) (I actually really respect Andrew Mason as a CEO)
There also be used to be companies who specialised in running small grocery shops in large US cities. They're gone, because the car made it possible to go to a larger, cheaper store instead.
But the concept of a grocery store hasn't gone away.