One specific example is that many major investors were highly skeptical about Theranos well in advance of the publications that exposed them. Why were they trading in private markets at such a high valuation when those skeptical investors could have been betting against them?
Pre-IPO startups, like all private companies, are not traded on any markets. They are private companies with shareholders and shares transactions are thus done 'manually' so to speak (you need to go out there and find a buyer, negotiate face to face, then handle the paperwork and payment).
Not sure if there are any regulation hurdle though.