General Guidelines on What to Sell online!

I've been researching SEO now extensively for about 2 months, bought some eBooks, and trying to share whatever i can etc.. All with the goal of starting my own online business.

But one thing I'm not quite sure is this -- how do I know what to sell? I know some folks say you should sell something consistent with your interest, hobbies, etc. And that makes sense to me. Put more succinctly, how do I know or how can I estimate whether or not a particular market/product will be profitable?

I have a specific product idea to sell. The #1 search phrase I'd ideally like to rank for shows over a million and a half (1,500,000+) searches in Overture last month. So that tells me that a whole lot of people are searching for this. But oddly enough it doesn't seem to be a crowded market--meaning I can't find too many sites actually selling this (at least not on the first few pages of the Google SERPs). I can't believe I've stumbled on a nice this good...I must be missing something.

provide infos if you can!

Thanks,