This is just my opinion, but I would say "no older than 2 years" is too harsh, unless you need all the features of Yosemite.
I have a top-of-the-line 2010 27" iMac for sale, and I know it's perfectly good running most features of Yosemite because I was using it daily until a little more than a week ago. But it's double your budget, and a good deal at that.
I'd say four years old is OK, five years old is pushing it, more than five years old is sketchy. But a lot depends on what you need to do with the Mac. Four year old Macs don't support a lot of the new functions of Yosemite.
Here are Apple's refurbs, for example:
http://store.apple.com/us/browse/home/s ... ac/imac/21
You can get a brand new base 21" iMac from Apple for about twice your budget.
http://store.apple.com/us/buy-mac/imac? ... tep=config
Here are a couple of Amazon listings:
My experience with buying an older, used, guaranteed
product through Amazon was a 24" Apple Cinema Display. It arrived poorly packed, in very used condition with cracked plastic on one end. It had clearly been dropped. Because the monitor itself was OK, we asked for a $25% refund and got it. That's probably because sellers through Amazon fear a bad report to Amazon. Frankly, I've seen too many postings about bad experiences with computers and eBay that I wouldn't buy from them.
I would say that "refurbished" has no relevant meaning in this context, except, maybe, that the seller swapped RAM or something.
Note that there's a new store in Sequim, just starting up, that specializes in used Macs. Their guarantee seems generous. They're local. I have to leave it to someone with a functional memory to give you contact information, but the couple was at the SMUG meeting last night.