Because ideas are not easily "excludable" ( http://en.wikipedia.org/wiki/Excludability ). How can you sell it if you're giving it away for free on the site? How can you buy it if you can't read about it?

That doesn't mean that an idea has no value, it means that it's impossible to transform into money. Or, if not quite impossible, that the transaction costs outweigh the value of such a market (I'm not convinced of the value of the original poster's idea:-)

BTW, I explained this a year ago:

http://journal.dedasys.com/articles/2007/04/26/ideas-are-wor...