Buying used gear is a GREAT way to save money, but it's not something I recommend for someone without at least basic camera knowledge unless they absolutely have to. If you want/need to, there are things to look out for: I would definitely NOT buy a used camera over the 'net or from anyone where I could not test it out... I know lots of people do, but it's not my thing.
Assuming you're going to buy it either from a dealer or local private sale then there are a few things to look for. First, buying from a dealer is less worrisome because it will usually come with at least a 30 day warranty, but a private sale will often be a better deal. Anyway, first determine a price, research Craig's List,
eBay and similar 'sites to see what the bodies you're interested in are going for. Next determine your maximum price and DON'T go above it; there'll be anohter one along shortly if this one doesn't pan out.
Once you find a camera you like, contact the vendor and ask him for the serial number. Call the local PD and ask them if it shows up on any lists as stolen/lost. If it does... If the vendor won't give you the serial number, WALK AWAY!
When you meet the vendor, examine the camera carefully, look carefullly at the outside. Is it clean and does it appear well cared for? Dirty gear is a sign of a careless user. Walk away. Next look carefully at all the corners and edges; look for any signs that it's been dropped or bumped. If there are ANY signs of damage, WALK AWAY. Now, since you already gone on line and read the manual for the camera before you met the vendor, you know how to turn it on; do that, (making sure that either your or the vendor bring a memory card and a lens) push all the buttons, operate all the controls; make sure they move smoothly and the functions work the way they're supposed to.
Take some pictures and review the results on the rear LCD - ensure that the results look 'right'. If they don't... you know what to do!
Ask the seller about the camera's history, how long he's had it, what he shot, etc... If all seems good and you're happy, buy it. If not... walk away.