Most pro take good care of their gear. But count does matter.
Here's are simple #s:
My d70 - over 120k frames on it. It was my first digital cam.
My d200 I bought refurbished With 10k on it and later traded it in for d300.
I bought d90 in 2009 - it was my backup and 2nd - today it has 33k on it.
D300 I traded in for 300s( brand new) march of 2010 today it has 65k on it. - it was primary until 3weeks ago and I got d700.
So backup or primary are subjective definitions bc things change.
Personally I wouldn't buy used from someone i don't know personally or wasn't referred to - it's a "trust issue".
Refurbished - been there, done that
Oh and all gear can brake
D90's shutter mechanism went at 12k
D300s hotshot was replaced at 4k.
That's why I was talking about the warrantee at least then u aren't spending more $ if problem occur.