A NYTimes article says
When employees leave start-ups, they often have the opportunity to buy stock that has been set aside for them at a low price. But if their start-ups have been successful, they also need money to pay taxes that will be levied on the increased value of the stock.
On this site JoeTaxpayer wrote
You got out of the company just in time. For whatever reason, the stock drops to $21 and at tax time you realize the $1M gain was ordinary income, but now the $800k loss is a capital loss, limited to $3000/yr above capital gains. In other words you have $210k worth of stock but a tax bill on $1M.
These two scenarios appear to discuss the same risk. One obvious way for the employee to protect themself (in the last example) is to sell the shares on the very day they bought them. Then they will be certain not to be left stranded with a hefty tax bill on shares possibly worth less than the tax bill.
But the same NYTimes article opens by stating that it is within the right of companies to stipulate what employees can do with their shares (actual, not options). I see how a company could add restrictions on stocks while an employee is an employee. But once they leave with shares, whether actual or exercised options, they are entirely free to do whatever they want with the shares. In particular, they can sell them to whomever they choose. Is that accurate or could a company put controls on shares owned by ex-employees, all the way until the potential IPO—restrictions that would bar ex-employees from selling?