Apple's computers are not the cheapest in the world, but are highly desired and as a result many people purchase their Macs secondhand through auction sites like eBay and Craigslist. Unfortunately in most cases these systems are old enough that their warranties have expired, so if something goes wrong after your purchase you might need to spend more cash to have it fixed.
Under the circumstances, it is best to give any used Mac a thorough check before you decide to purchase it. (Obviously, it would be hard to do this for most online purchases.) I've covered this before specifically for MacBooks, but here I'll address it more generally.
When you examine the computer you're considering buying, it's best to bring some tools with you:
- A FireWire drive
- A USB drive
- Headphones and a microphone
- A CD-ROM disc, DVD-ROM disc, or SD card depending on the computer's ports
- An Ethernet cable and an Ethernet device (a powered hub or router)
- A working external boot drive with an updated version of OS X
With these tools in hand, you can quite thoroughly check the used Mac's hardware to make sure it is functioning properly.
- Give it a physical
The first thing to do with any Mac is to give it a once-over to see if there is any physical damage. For laptop systems, be sure to check the display's hinges to see if they hold the lid open or if it falls shut. Apple's laptops should keep the display in whatever position you set, even if the system is slowly rotated around. If the display is loose then it will fall shut. Check any latches and hatches to see if they function properly, and check any built-in buttons (keyboard keys, power button, and so on) to see if they are loose or otherwise not working properly.
- Check its ports and drives
Ports on systems can get worn out or blocked by foreign objects, which can be a problem when trying to use peripheral devices or other hardware. Use a flashlight to check for blockages or bent pins, and also check their functions. If you are meeting someone to purchase a Mac, as mentioned above, bring a USB and FireWire drive, CD and a DVD discs, and headphones and a microphone with which to check its ports and drives and see if they work. Unfortunately there may be ports like the ExpressCard ports on some older Mac systems that you might not have an immediate means of testing, but you can at least check them for blockages.
- Check its innards
If you can open the system (mainly for Mac Pro systems or laptops with access hatches), do so and inspect the insides. If you can see any obvious damage or areas with dried liquid residue then be wary of what the system might have been through.
- Check for a firmware password
A set firmware password can prevent you from accessing alternative boot environments for troubleshooting purposes, so be sure to check if one is set by holding the Option key at startup. If you are presented with a boot menu without any password prompt, then the system does not have a firmware password set. If the system was made before 2011 then the firmware password can be reset by altering some aspect of the system's hardware (for example, by removing a RAM module). You can also reveal the firmware password by logging in as an administrator and running the following command in the Terminal all on one line (thanks to MacFixIt reader "davidwtbuxton" for contributing to this command):
python -c "print ''.join(chr(int(c, 16) ^ 170) for c in '`sudo nvram security-password`'.split('%')[1:])"
However, if the system is a newer one from 2011 or later then Apple will have to reset the password, so be sure to check for the presence of one and have the original owner clear it if possible.
- Check hardware function
This step is the most important part. While dings and scratches might be tolerable, if the system is experiencing a hardware fault (even if the fault is not immediately noticeable), then it could cause the system to run into errors down the line. Check the system's hardware by booting to the Apple Hardware Test suite and running a full check on the system; see this article for more.
I highly recommend making sure that a Mac passes the Apple Hardware Test before you purchase it. There are many temperature, current, and voltage sensors throughout the computer that stabilize its function, and if they are not working or they are reporting errors then the system might not last as long as expected, or might give you frustrating problems such as system fans always running at their highest speeds. In addition, be sure to test the system memory with the Apple Hardware Test suite, as bad RAM can lead to seemingly random crashes and hangs.
Lastly, if you have a drive-testing suite such as Drive Genius, DiskTools Pro, or TechTool Pro, then use these to perform a surface and integrity scan of the hard drive to check the storage media for problems. Regardless of the drive's formatting, if it cannot hold data reliably then it will not work properly in any computer. If you do find bad blocks or other problems then it's not the end of the world, as the drive can be swapped out for a new one; however, this will cost you a bit more to do, so consider it as a factor in the price.
- Optionally, check the software
I strongly recommend if you're purchasing a used system that you wipe the drive clean by partitioning it yourself and reinstalling OS X from scratch, since not knowing what has been installed on a system poses a security and stability risk. However, there are times when this might not be possible, in which case do boot the system and use Disk Utility to check the drive and filesystem for errors. At the very least be sure to create a new administrator user account for yourself and remove any prior accounts, but consider wiping the drive and doing a fresh OS installation when you get the chance.