In fact what I would do if I ever bought a used camera (other than online which I did recently and because of the source and the community I am connected to I knew it would be fine) ... but not on the internet here is precisely what I would do.

1) Locate by research the type of camera you would like to purchase
2) Study what you can on it to know what to expect of it when it is in your hands. Look at picture galleries that aren't overly processed. For example, dpreview.com often has photo galleries from various cameras they review. Get onto forums and find out what people are saying about the camera, good bad and everything in between to ensure that 1) it fits your mold and idea of what you are looking for and 2) you have some familiarity with it when you go to test it.
3) Find someone selling it used that is within vicinity
4) Ask the person if you can check shutter actuations and do this ... how to access each camera's actuations is available online via google searching. Per literature each shutter is rated at X amount of actuations. The lower the better! Unless of course it was just serviced by the manufacturer and the seller has proof.
5) Test it out, bring yourself a memory card and take some pics. Make sure to check the various buttons, knobs and internal functions to ensure it ALL works.
6) Either thank the person and let them know that you will call them in an hour or you can get it right there. You can then view the photos you took once you get home or if you are like me you have a laptop and can plug them in immediately.
7) Restart at 3 if you aren't satisfied with THAT camera or start at 1 if you want something entirely different.

Optional 8) Best to do such buying/selling in public places such as a coffee shop or something.

Good luck!