Purchased two, a Canon GL2 from another guy in the business, and an older model S-VHS camera - Panasonic AG-460, from someone also in the business, both found on sell/trade video production forums. Both times we each were strangers and had to extend a degree faith money vs equipment being what it said it was. Both times I have been treated kindly by the gods and received full value for what I paid.
I knew what I wanted both times, and knew the prices were reasonable (not giveaways, but significantly less than new), but beyond a bit of extended dialog with both sellers, e-mail, posts and phone, I don't know much more about how to research people.
I might have a bit more trouble trusting used purchases from anyone other than people in the business who generally have taken decent care, even if they have worked that puppy to death to get full R.O.I., but generally speaking, if not purchasing from a huge production facility or a cable or broadcast facility, the independent user/owner, IMHO, is a more trustworthy soul on all levels, and the equipment isn't likely to have as many miles on it, or been abused by uncaring employees. Maybe. Sometimes. So I'd be inclined to buy from an individual who has been or is in the business.
Many, like me for example, are offering decent prices on equipment that still has GREAT life expectancy and service because we are moving into a different line of equipment, upgrading, or somesuch.