Q. Where do I find my first customers and how do I build a critical-mass of traffic?
Once you have an eBay store, a content site (with AdSense and Affiliate offers) or just a basic website promoting your own services, go to friends, family and old co-workers. Ask them to visit your site and ask for honest opinion.
Very Important: Don't tell them this site is yours. If you do, you're not going to get honest opinion. Instead, say you're looking to buy this site and you want to get their opinion if this is a site they would visit again.
Having a site with high stickiness factor, where you offer enough of a value proposition to website visitors to come back, is very important to building a sustainable online income.