My guess this subject has been covered several times around here. If there is a good link, please direct me... I usually keep my Mac about 3 years and then sell them for a new one. I'm currently using a MBP (see signature). It came with Snow Leopard and I am considering upgrading it to Mountain Lion. My question is when I buy an OS upgrade from the App store and apply it to my MBP, when the time comes to sell, can I leave Mountain Lion on it and person who buys has a way to reinstall Mountain Lion from scratch? Or until they buy it for themselves, would they have to revert back to Snow Leopard on the original install disk? Thanks!