you personally would have to find somebody that wants to buy them - it really is as simple as that - they will be illiquid until Monzo is a listed company and that won’t happen for 5-10 years or an institutional investor might want to hoover up small shareholders (unlikely) , or they could go bust and you will lose your stake, if they do get to listing on the stock exchange, the company could be the next Facebook - until they list they are basically worthless bits of paper unless you can find somebody to sell them to
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