Joel wrote about this a few years ago (2000, before the creation of StackOverflow):

  If you're going into a market with no existing
  competition, lock-in, and network effects, you better
  use the Amazon model, or you're going the way of
  Wordsworth.com, which started two years before Amazon,
  and nobody's ever heard of them. Or even worse, you're
  going to be a ghost site like MSN Auctions with
  virtually no chance of ever overcoming ebay.

Extracted from: "Strategy Letter I: Ben and Jerry's vs. Amazon": http://www.joelonsoftware.com/articles/fog0000000056.html