>This is a common mistake: building a product based on assumptions, and not on the market's demand. I've learned not to add a feature before I have someone who paid an advance.

I first came across this sort of advice here:

The Montana Mogul: RightNow CEO Greg Gianforte (Part 1)

http://www.sramanamitra.com/2008/07/31/the-montana-mogul-rig...

Interesting story. RightNow was later sold to Oracle, IIRC, for a good amount.

Greg used a similar technique to what you advise.