I agree with Betty, buy an older version, CS3 or CS4, unless you are running Mac OS 10.7 or 10.8. Don't bother with the Extended version, it's unlikely you'll need the extra features. However, make sure that you're buying a full version, not an upgrade, and that it's the correct computer version (Mac or PC). Most important of all, make sure the previous owner has deactivated it on their computer, which allows them to sell it, so you'll be able to install it. They'll also need to give you the serial number, tho' that should be on the box, or DVD case. Here's a link that goes over deactivation:
If you are using Mac OSX 10.7 or 10.8, you'll have to use CS5 or CS6, earlier versions won't install, as they're not 64-bit clean.