A helpful tip for those in the market for a used Apple Macintosh computer. Whether you are buying from a retailer or individual seller on eBay, be sure to ask if the seller knows the computer's admin password before buying.
The admin password is needed on most all Macs to install a number of hardware devices including wireless adapters. You will be prompted for this information during the installation process, and if you do not have it, the only way you can reset it is with the Mac OS installation disk (which is usually not included with used computers). If the seller is the first and only owner of the system they should be able to provide you with the information. If the seller is a retailer of many Mac systems, if they do not know the password they should be able to reset it for you using an install disk before shipping your system.