Whats the difference between the rank and the rating? I doubt that both of them are needed in the system.
Other users rate you, and you are ranked automatically.
Do you mean that a group of users can rat out some good competitor, giving him low rating?
Theoretically, yes, but in practice, he or she can ask support team and improve the situation quickly. Offenders will be punished for sure.
I heard that its possible to buy rating. Many advertising-related projects provide such an opportunity.