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Buying shares in a company after you quit
Insider trading is when you buy or sell an investment based on material, non-public information that gives you an unfair advantage over the rest of traders in that market.  Working for a company is one way that you might have such information, but whether it is insider trading is not contingent on you working there. You could use that information a long time after leaving the company. You don't even need to have worked there. If a friend/relative gave you non-public information because THEY work there, it is still insider trading.