The Founders Club enables founders and early investors in later stage companies to sell stock in privately held companies for cash. In the past founders and investors did not sell stock until the IPO, because it was impossible to sell before the definitive exit of the company. Now it is possible.
Later stage seed & Series A & B stage investments are managed via Rubicon Venture Capital. Check out Romans' book THE ENTREPRENEURIAL BIBLE TO VENTURE CAPITAL: Inside Secrets from the Leaders in the Startup Game.