This might be helpful,
10 things to test before buying a used DSLR | ADIDAP
Checking the sensor for excessive dust, hot pixels, and the number of shutter clicks would be the key things for me. Don't get too hung up on the shutter clicks, most of these cameras will be replaced due to advancing tech long before they wear out.
I live right around the corner from KEH and you can't go wrong with them.
The classifieds section over at Fred Miranda is also full of nice gear and reputable sellers.
Buy & Sell Photo-Gear - FM Forums