All Character sales must abide by the Rules here. In accordance with the rules all Kill Rights against the character being sold must be disclosed. If this is indeed the case Buyer are within his/her rights to demand compensation or if no agreement can be reached between the two involved parties, then the Sale may have to be reversed.
I suggest you discuss the option for compensation or if not then a support ticket may be the only way to resolve the issue (may even be classified as a Character Scam).