I created one of these. My company used to actively develop for a particular market. Today we no longer develop for this market, but it's a big market and the large players have affiliate programs. We utilize these affiliate programs--they provide XML feeds and the like, and our site's backend combines them together and figures out the overlap--to run our own site in the space. The market players are quite competitive, and feud over exclusive product distribution, so ironically I think we actually have the largest inventory of any single non-affiliate site.

It took about two months of my time to do the original website, much of it spent on SEO optimization and traffic generation strategies. Today we earn ~$8k/month on the site with an upkeep of ~30 minutes/week.

This worked primarily because of deep knowledge of the market (from back when we were developing this kind of software). I don't think an outsider would have been able to predict hit products far enough out to lay the proper groundwork for SEO/etc. When I was actively creating the site site you could search for "Product X" on launch day and we'd be #1 or #2.

Note: I also launched the site on our company's domain name, taking advantage of our existing inbound links. SEO from scratch will take a year or more to really see predictable traffic levels.

So, yeah, it's definitely possible to create a cash flow muse; I think I could build another one today in around the same amount of time.

It would be hard to do from a totally cold start, though; I'm not sure you could say "you know what, I'm going to make money by creating a local real estate website", or whatever random topic, and actually make it happen. But if you had friends/family in the real estate business, or somehow had accrued expert knowledge in the area, you'd be off to a strong start. If you're smart about the area you pursue you should be alright...