There are good deals out there, but I seriously doubt this is the one. I once met someone selling a 300mm f4L IS for $800 in like new condition on Craigslist. We met at a public place (her place of business; a bank). I walked through two feet of snow to get there, she felt bad, and sold me the lens for $700. That's a 50% discount!
Indeed. I, too, picked up a 300/4L IS for $700 (and after getting the 100-400L, I sold it for a reasonable profit
). It was quite a decent lens:
Rebel T1i, EF 300mm f/4L
IS USM, 1/500 s, f/4, ISO 200
I've also bought a couple of other lenses for ~50% of their new cost - a 200/2.8L II (subsequently sold after getting the 70-200 II), and an MP-E 65mm (still have it). As a rule, I won't pay more than 70% of new price for a used lens.
So, there are deals out there - but you have to be careful.