Why not buy an Apple Refurbished machine from Apple's online store. They come with the same warranty (1 year) as a new machine, are able to buy AppleCare (extends the warranty to 3 years) and are virtually identical to a brand new machine. You can also save up to 20%. To locate the Refurbished section open the online store, scroll down and look on the left for Refurbished Mac. You didn't complete your profile so we have no idea what country you are in, in US the address for the refurb store is: http://store.apple.com/us/browse/home/specialdeals/mac BTW , what they have in stock changes very quickly so once you decide then buy then buy it otherwise you will probably lose out. Also I would NEVER buy a Mac (new or otherwise) from EBay, you have no idea what you are really getting.
I am not big on buying any Mac from eBay.
You simply don't know what you're getting into, some sellers don't include the Mac's original OS discs and no one on eBay can offer any acceptable warranty period.
If you are looking at purchasing a "newer" used iMac, there are reputable Mac resellers that sell used Macs at slightly higher than used market prices, but you generally get a used Mac with some warranty and/or extended warranty time. The reputable resellers usually spend time to quality check the used Mac for reliable operation and condition. Better resellers also have a warranty period of, at least, 30 days. Some resellers have a 90 day warranty and offer an extended 90 day warranty. So, you'll have 6 months or warranty time in case something goes bad with a Mac within that time.
What is your spending budget for a newer iMac?
If you are looking to upgrade to a newer, used iMac, look at models from 2008 to perhaps 2010-2011 models.