If you're planning to buy a Used Mac now while also intend to upgrade the operating system to OS X Mountain Lion in the future, you'll have to be cautious.
Like current OS X Lion which cannot be installed onto Mac that runs processor lower than a Core 2 Duo, this Mountain Lion cannot be installed onto Mac with lower-end Intel GMA X3100, GMA 950 and ATI Radeon X1600 graphic card.
This simply means that all pre-Unibody Macbook White or Black will not be able to run Mountain Lion. MacBook Pro 2.33GHz or older will not be able to install Mountain Lion too.
Long story short, if you plan to use OSX Mountain Lion in the future, check your system now.
