No, I am reasonably certain that buying/selling/trading is generally not allowed here. There's just no way to protect sensitive private data with this particular format.
Apple's website does have a refurb section, and the one nice thing about Apple refurbs is they all carry the same 1 year warranty as new units. The link is a little difficult to find, but it's most of the way down on the left hand side of the Apple online store. IIRC, it's sort of blueish in color.
You could certainly get a better price on the likes of Craigslist or eBay, but then you have to try and vet the seller, you never know how well the system was (or wasn't) treated... There's always even the chance that it will be stolen merchandise. None of which you have to deal with if you buy direct.
The Mac Mini is a good low investment option to get you started too. It's powerful enough to be a suitable desktop system for most people, and assuming you already have a monitor, mouse, keyboard, etc, you can just reuse those. Though if the monitor doesn't do DVI, you'll need the Mini-DisplayPort to VGA adapter either from Apple or you can see what Monoprice.com has in the way of cables/adapters. If you find that the Mac platform really isn't for you, then the Mini minimizes your investment and it will always make for an excellent HTPC device. And if you become a convert, then you can look to upgrade later.
I am looking to purchase my first Mac, but might be interested in used/refurbished, etc. Is there a section on this forum that address that?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic