maybe send them a message along the lines of "I am sorry to hear that your eBay account has been used without your permission.  I will be reporting your account to eBay as a hijacked account to prevent you and your account being used for unauthorised purchases. I hope this will not inconvenience your eBay activities for too long, but unauthorised use of an account is a serious matter".

See how that works . . . . and report the account