another thing you want to check on, especially if it is a reef system you desire, or may desire in the future,
ask them if they ever had any disease or parasites(ie ick), and if so what meds did they use
then check on the meds, if they had copper in them, nix that system, otherwise, fill it and see if it will hold water before you buy, if they want to sell the tank, they will fill it with fw, and show you that the pumps work and that there are no leaks, or if it is full, tell them not to tear it down til you get after there, even if you have to help, it is to your benefit to see it operate