Threadless is a t-shirt e-tailer and the cool thing is that they crowdsource both supply and demand.
This is how the site works, from Wikipedia:
Members of the Threadless community submit t-shirt designs online; the designs are then put to a public vote. A small percentage of submitted designs are selected for printing and sold through an online store. Creators of the winning designs receive a prize of cash and store credit.
On the supply side the community submits designs to the site and then votes on which designs should be produced – so Threadless crowdsources the design and selection of their merchandise.
And then the reason the whole things works so well is that people who have voted for the designs are then much more likely to buy them, which is what I mean by crowdsourced demand.