Sure, you can do whatever you want (buy, sell short, or whatever) assuming that you have enough fund ($$$). For example: It's ok for you to own 100 shares of XYZ and also short 100 shares of XYZ. So far so good, no problem there! It doesn't matter which direction XYZ goes, you won't make any money and you won't lose any money either. Now, let's say that you decide to cover 100 shares of XYZ. Most brokers won't allow this kind of transaction; first they will make you sell 100 shares of XYZ before you can cover it. In the end, you would end up not owning any stocks; therefore, it would be better to sell all your XYZ holding before shorting it.