Business isn't easy; you'll need every advantage you can get. My advice (and what I'm personally doing, after being in almost the same situation as you) is to build a smaller business first, specifically something that does not require a network effect to be useful. In other words, a product that is useful to a single customer without any other users. See stuff by 37signals and most SaaSs in general for examples.
The end point is really that business is hard, and it's infinitely harder when you go after something huge like eBay on your first try. That's not being bold, it's being foolish.
Why not build something smaller to learn more about business, make a good bit of money from that, and then take on eBay? It is quite a bit easier to build a 100k/year company than a 100million/year company. Plus, you'll have money to bootstrap your company.