From building a marketplace at Ridejoy (YCS11), I can tell you that this early market building is very unique to each team/product/market and also one of the most valuable "secret sauces" for the company. Anyone who really knows probably won't tell you, and most people who might write a response are mostly guessing. At least that's what I think.
These marketplaces share a lot of core business similarities to that of the daily deal market as early as four year ago, no? At scale (depending upon which niche you choose for your site) will require money in to make "money out". No one in this thread seems to be highlighting this with the exception of those sites listed, little VC dollars have been invested thus far.
It's a serious question and one that's not secret sauce, I'd argue. The execution of your business and that of any sticky viral hooks you can build in will leave you at the winner's circle years from now.
