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Could an ex-employee of a company find themself stranded with shares they cannot sell (and a tax bill)?
they are entirely free to do whatever they want with the shares. In particular, they can sell them to whomever they choose No. Restrictions on who can sell when and to whom are a common thing with startups. "Publicly traded" companies are regulated in a much stricter way than private companies, so until the IPO the sales are limited to the OTC markets. But even that can be restricted by bylaws - for example ownership can only be limited to a group of investors approved by the board. As an employee - your grant was approved by the board, but when you come to sell, the buyer was not and the company may not agree to vet them. Bottom line is that it is not illegal to impose all kinds of restrictions on what the employees can do with their shares, as long as the shares are not listed on a public stock exchange (even after the company goes IPO with one class, other classes may remain restricted).