Okay, I think I understand what you want to do, but I'm not sure how practical it is. It's simple enough to take out your current 500gb, install the 250gb and put a fresh install of the operating system on it. Then you can swap back to your 500gb and use it for your daily use. At this point you have two functioning drives that will work when installed into the machine. What I mean by "not practical" is the idea of swapping out the drive every time someone wants to look at the computer. I don't recall off hand if the 1st gen Aluminum Macbook had the easy swap on the bottom or if it required that you fully take apart the machine. If it is the easy swap hard drive then this would make sense and could easily be accomplished. However, if you have to take apart the entire machine to swap out the drives then I would let people browse the machine with the 500gb in it and not do the full swap until you have secured a sale. Opening up a machine like that over and over is just a risk.