Well, If your buying second hand, you have to be slightly more careful, I recommend buying it at a store where if you can, try it out, check for these things: Wear and tear, scratches, dents, and open to see the sensor and check of signs of wear and tear because most SLRs are good 100,000 shutter activations. I bought my Nikon D70s at a local Ritz Camera and they were kind enough to let me handle it and take a few test shots. Bottom line: I recommend buying it at a store instead of online
P.S. If you want to take test shots, make sure you are ready to buy and bring your own lens.
Hope this helps a little