A close friend bought a used iMac from Apple a couple of months ago. He said the machine was like new when he got it. If he had not purchased a used one, he would have assumed it was new. Just 1 personal experience. It was his first Mac also.
For safety's sake, definitely an Apple refurb. eBay, Craiglist, etc. all tend to be a crap-shoot, even with high feedback sellers. But then again, there's plenty good to be had on eBay - I've purchased several used computers on eBay over the years (both Apple and other PC brands) and only had one real issue; and that issue was handled to my liking by the seller. I've also sold ALL of Macs from the past on eBay, including G3/G4 towers, a G4 iMac, a 17" white iMac, a Core Duo Mac Mini, a PowerBook, a Macbook, and two Macbook Pros. Point is there are good, honest sellers (ME!!! ) and plenty that aren't.
My advice: If an Apple refurb is out of your price range and you need a new(er) Mac, eBay is definitely worth a look-see. Just do your research and practice safe buying.
Over the past several years I've had great luck with both Mac of All Trades (in Florida) and PowerMax (in Oregon). I've found them to be really conscientious about what they sell (both before and after the sale) and no, I don't work for them either. I'm in Texas.
What about Gainsaver.com? Their prices seem abnormally low. Here is an example. It's about $200 less than the same item from Barnes and Noble. It's refurbished from both retailers. Is a refurbished computer any good?