If the price is too low to be true, than it is almost always a scam.
I saw a 70-200mm f/2.8L advertised on our local craigslist for $400 last week. I dismissed it as a scam, but after reading it, it was too well written for a scam. I contacted the owner, and he told me how he bought it new several years ago for film photography, and saw a haze on one of the internal lens elements. He sent it to Canon, they cleaned it, but could not completely remove the haze. It was not a fungus. They told him that it did not affect the optical performance according to their testing. Then he lost interest in film photography and put his gear away in 2005 never to be used again.
I went out to look at it, and it was showroom spotless, as was his really nice home. He said he would consider a offer, and since I had not tested the lens, he accepted $350. Then he proceeded to bring out his other equipment one piece at a time, all in the original box and looking unused. I ended up buying it all for less than the usual scammer low prices. He even had the original B&H receipts.
I, have held the lens to the light, several different ways, it is remarkably clean and clear, and autofocuses accurately. I can't see a haze, and the images are sharp and detailed. I'm tempted to sell my 70-200mm f/4 IS and use the f/2.8 non IS for everything. With my new 5D MK III, I might be able to boost the ISO enough to get fast shutter speeds even in low light.
Wow, you got it good...