eBay is a really popular site. This is a credibility issue. Does eBay have it? Yes. It makes sense to deal with sites that have high credibility, and by adding adsense to your pages.

I would not buy such a web site because if it is making money on an ongoing basis why would they be selling it?

If you want to learn about how to make money from the Internet then I would suggest you learn from someone who is already making money from the Internet. If you don’t know someone personally then have a look at this information http://www.clickdigitalproducts.com/featured/ebooks/super-affiliate-handbook.html which I think is a good place to start.