Hi Caspere,
love your comments! Hope you find a really good deal for your Canon 28-70mm f/2.8L lens. I actually seriously thought of bidding again for your lens after i backed out cos you seem to have been a serious photographer for the last 10 years and you probably tested out all your lenses carefully already.
Which brings me to a thought: If you see a CSer with many years of experience selling a lens he's used for a long time, wouldn't you trust his lens that he's selling, more than say someone who probably is newer and less experienced, since he would probably have worked out whether its a good copy or not?
If he has held on his equipment for a long time, beside the above said, it can also mean that he use it more often than those who own it for collection purposes. So, the collectors' items may be newer and less worn out?
Seriously, I am almost embarassed to say I don't do "technical" tests for my equipment. Most, if not all, my experiences are with real world shooting. Of course I will check with people/friends, and occassionaly refer to "tests" & "reviews" done by other people like photozone/photodo/dpreview. I used to frequent photo.net often, but I was not as serious as the people there about their art.
Of course I must say I am pretty lucky in the sense I have yet to end up with a "bad copy" camera/lens, but everything there's always a first! Like ... I have yet to use my 24-70L so cannot make any remarks yet.
Then again, I read of people who were constantly given lemons; either the body/lens is not calibrated, or backfocus, or with multitude of reasons. I must say I am thankful for their experiences as I read them, and kept aware of developments. Indeed, my Sanyo 2500mh batteries couldn't hold charge like most people's, and it went back to Sanyo.
Trust is only as good as money is green.