a) Person A --> granted provisional access to the system -> legal
b) Person A sells the access to the system to person B -> illegal, misuse of access by sale of credentials or access codes.  Can fall under access fraud. Also illegal if "making available of" (distributing) credentials without authorization of site owner per ToS (again provisional access requires that rules be followed)
c) Person B accesses the system -> illegal, computer fraud, access fraud, and theft of service.