It goes to how well you imagine the person takes care of their equipment.
I always insist on pictures (close-ups) of the seller's teeth. If they don't take care of their teeth, do you really think they'll take of their equipment?
I also like to pictures of their lawn. In my experience a well kept lawn -- a well kept stereo rig!
I also ask for pictures of the family. If the kid(s) look unruly or even a little radical, then no will buy. If you can't properly raise a kid, then what are the chances of maintaining a turntable.
Records of auto maintenance are extremely useful in judging a potential seller. IMO a man who rotates his tires every 8,000 miles is a man who can be trusted.
Finally, if after all this info I'm still a little unsure about buying from someone I give them the opportunity to write a 400 word essay on the subject of "Why Should I Buy Anything From Them".
Once I have all the requested info I can then start the negotiation on price. You just can't be to picky about who you buy used audio equipment from on the internet.