Used from a reputible dealer is little different from a refurb. Items are often inspected in the same way as refurbs, might be repaired if needed, same as a refurb, and usually comes with some moderate warranty period, though it's usually less than a refurb. Adorama, B&H, KEH are all reputible and reliable used gear dealers.
Used from a private party is generally a bit riskier, but prices are usually lower. As stated above, you have to know what you are buying and be able to evaluate it quickly, if buying in person. Assume there is no warranty, since you'll likely never see the seller again.
If buying from private parties off the Internet - such as certain auction sites - you sometimes have limited recourse if something isn't up to snuff. Sellers at the afore-mentioned auction site do in some cases offer modest right of return, though you might be out the cost of shipping. And there are means of contesting an obvious rip-off.
Over the years I've bought new from dealers, used from dealers, used from private parties, and refurbs... generally not had any problems. Only had a rare issue or two.
***********
Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
