If it were me,I'd buy a new one.....some things are ok from e-bay,but you could be just buying someone elses trash by taking a chance on a used one.....just my opinion.
You're also gonna want to know why this happened.There are several posts on here about this problem,and there are several different solutions.My solution was to go with the Renthal 14T front sproket(the Renthal stayed away from my case more than any other)...AND I went with an RK MXU chain,which is considerably narrower than all the other o-rings,and has 9000lbs tensile strength....which is also very good considering I've always used Regina chains,and their tensile strength was around 8000-8500lbs.I put the RK on this past June,and have'nt had to touch it yet....and I have about 1/4in clearance from my case now.
Foxx