A small update so far.
No, the seller wouldn't take it back. I have his adress, phone number , name of parents etc so I have a few more cards to play if it doesn't work out
No, I will not accept a $2000 paperweight.
The lens is still under warranty so I have shipped it to Canon, arrived there today, so within the end of next week I will probably know what the damage is. The seller said he would split the bill if the fix isn't covered by the warranty after all, I however thinks he will pay the whole bill.
I went to my local shop and tried two other copies of the lens and they were both SERIOUSLY nice and VERY equal, so when it works I think I'm going to use it a lot.
On a side note I shipped away my 85 L today for the third time in 6 years. I miss it already...