Which model Macbook do you have? Which OS version is installed?
http://support.apple.com/kb/HT1635 How to identify MacBook models
http://support.apple.com/kb/HT1159 Mac OS X versions (builds) for computers
You state that the Macbook is old yet you posted in the ML forums. Did the seller provide you w/the system DVDs that originally came w/it? If not, get them from the seller. "Old" Macs are not suppose to be sold w/o them.
You need to wipe your MB securely. You do not know what the seller has installed.