It's better to ask the person to do most of the test shots and give a link to examine it. And then you go and check it out physically. I bought used DSLR 3 times and new camera once. The used ones are Nikon D70, Pentax K-7 and Nikon D200. My procedure is as follows.
1. Ask the seller to have test shots in the list you want. Like, scene, people, flash etc.
2. Check the pictures and analysis them online. Like shutter count, overall sharpness, color etc.
3. Asks for an original pictures for you to play with in post processing. JPG or raw as long as your mail box can handle. gmail can handle big size files.
4. Have a phone conversation. Many the time, you can judge the personality through a real talk. Many headache we may have with used gears are personality related. Like honesty.
5. Ask as many questions as you can.
6. Take the chance of failure by negotiation the price reasonably. $10 or $30 won't make either party to die but that related to the overall happiness about the transaction ^_^. Consider that as a lunch with a friend.
In the US, most time you'd be fine. I met many honest person in the world in pentaxforum and photo.net. Another trick is to buy gear from people who shoot German glasses. Most time their non-germany glasses and gears are just sitting there and gathering dusts.
Have fun and enjoy!