When we talk about online business, people usually think about creating your own website and having an inventory in your garage. There are other means to have an online business without having to spend for your own website. You can even use other people's website.

Ebay

If you still haven't heard about ebay, then you're probably in a cave somewhere. It is one of the greatest innovations e-commerce. It allows anybody to post products and sell in the internet. You don't need a website for that.

Blog monetization

Start your own blog and monetize it using adsense and text link ads. There are sites out there that talks specifically about making money using adsense and similar ad publishing revenues.

Domain Parking

There is actually a group of people investing in domains and registering expired domains. What they usually do is to "park" these domains at some PayPerClick companies which in turn serve ads on these domains, and the company and the domain owner split the income. Some people buy thousands of domains for this.

People profit from ads, and from sale of the domains. Wanna know how much some of them are selling their domains? Check out sedo.com