I think by the very fact that there isn't another eBay, to use your example, shows that there are actually many barriers or entry. I believe your example is confused ie. the webmastering v's the business. Web development on a massive scale does require fantastically talented people and a technical infrastructure, but ultimately the web side of things is quite commoditised and quantifiable. On the other hand the business acumen and finding the right people to build a business over time is so much harder the tactical implementation of a site.
Of course any business web based or not could be challenged, but finding the right group of people to do that is harder then it sounds.
