If your talking on the open market, and not employee options its' really an issue between you and the SEC. If you were an insider with material information, you can't trade until such time as that information is made available publically (e.g. if you have earnigns information or M&A information and you leave before the event, your still have inside information and can't trade). Once any inside information you have is available to all or outdated (for example you thought they were going to buy company X, you quit, the deal never happens, at some reasonable point you can conclude the deal fell apart and is no longer inside information) you should be free to trade.