Sorry to hear about the problem you are having op. But not all second hand (or even multiple hand) deals end up like this. My camera and my 70-200 are 2nd hand and they are flawless.
I prefer to buy from folks who I know through the forums - here or on fm. It typically helps to ask your doubts on condition and return etc. and then make a deal. Most folks here sell coz they have realigned their setup - not to get rid of defects....
I'll probably end up buying this lens new than save 200 bucks.
I told them it worked fine before I sent it to them and sent pics to them to prove it. They gave me a brand new 135L and that issue was solved. Not long after that I baught a used 24-105L off an excellent POTN member and had calibration issues with it, this lens was even calibrated by Canon a few monthes back and the repair bill was included in the sale. What I think happened was it was bumped around during shipping and this was enough to put it out of calibration. Canon again wanted me to send in my camera with the lens but I said twice was enough. The lens came back over a month later and the backfocusing issue was repaired and it works great now.

