A client asked what the best approach would be to buying a few used iMacs for her family for Christmas, to which I replied:
I would not use eBay at all, I would go to Kijiji and Craigslist. That way you can actually go and see the computer before buying it. Yes, you probably will pay a bit more for it.. but it will be less headaches in the end!
As for which models to buy, go download the Mactracker app for iPhone or Mac, and use it to look up and compare with what is for sale. The models you want to avoid are ones that do not meet the requirements for running Lion:
http://www.apple.com/macosx/specs.html
Don’t worry if the iMac you are looking at does not already have 10.6 or 10.7 on it, just look at the CPU speed and RAM, the OS itself you can update later since you already own copies.
Once you find a likely iMac, ask the seller for the serial number. If they are unable or unwilling to provide it, move on.. once you have it, put it into this page:
https://selfsolve.apple.com/agreementWarrantyDynamic.do
Which will return the warranty and service details on the iMac, and this page:
http://support.apple.com/specs/
Which will return the specs on the iMac. Oh, I also ask the sellers about pets and smokers.. since my kids have allergies, and I can’t stand the smoke smell. My biases, it’s up to you to ask in advance, or decide when you get on site. Once you have those details you can decide if it’s worth looking into further. I’m assuming you’re going to read all about how to avoid scams, so I will not go into any of those details.
Next step? You’ve contacted the seller, have brought a friend along, and are meeting the seller and looking at the iMac. Ignore the iMac for a minute and look around, that will tell you a lot more than looking at the iMac itself. Once you are back at the iMac, boot it up and make sure it is the same one you were told about, basically check under “About this Mac” for the serial number, CPU and RAM details. If you have a usb key you can check each USB port to make sure it works, and if you have a DVD you can make sure the drive works. That’s about it.
Once you have it back at home use your handy OS X 10.6 install DVD and erase the hard drive and reinstall the OS from scratch. Unless you have a 10.7 installer.. at which point you should use that instead!
Have fun, and feel free to get in touch with me if you have any questions!
p.s. RAM is cheap and easy to upgrade on an iMac, so you might take that into consideration as well, low RAM in the iMac might be a benefit as you can get a good deal for it and then add RAM yourself. Not sure what kind of RAM you need, and what the costs are? Take a look at http://canadaram.com for details.