KEH has 5D classics for $550-650 fairly regularly, so I have no idea why you're making offers around POTN for one.
The only deal I've ever done over POTN was local pickup.
Outside of the marketplace, POTN's busiest forums are all wrapped up in the most tiresome minutia:
"What lens should I buy?"
"Is my camera focusing correctly? I really want to believe it isn't the most likely explanation--me."
"I dropped and broke something expensive because I'm an irresponsible scatterbrain."
and my personal favorite,
"Why doesn't everything spew rainbows when viewed at 100%?"
Seriously, if you go to the EF lens forum right now, those topics will be dominating the front page. I generally stay out of the lens and camera forums unless a serious technical question pops up.