It appears to be impossible to sell intellectual property.* The only way is to fasten it to something you can sell, like a physical book or disk. Failing that, one attaches it to advertising, which can be sold, but the problem with that is that nobody wants to watch the commercials.
Maybe that's why the Facebook IPO was disappointing: Facebook hasn't figured out how to force us to look at the ads.
*"She still believes in classical economics, the allocation of resources under scarcity. Information doesn't work that way."--Charles Stross, Accelerando
Link thanx to Making Light, which makes other good points about it.