Shares of Stripe, heralded as one of Silicon Valley’s most valuable private companies, have plummeted on the secondary market in the past ninety days. At the beginning of this year buyers on the secondary market were paying $81 for a share of the payments giant which was founded by Limerick-born brothers Patrick and John Collison in 2010.  Those shares are now being traded at under $50. While it could be employees of Stripe trying to sell some shares and take money off the table, the secondary market is open for trade by any shareholders including hedge funds, private equity funds…