Is this statement true in Canadian agencies and contract law?

"It is an agent's obligation to act on behalf of the principal and fulfill the best interest. For instance, A hired B to sell his car, A want to sell it for at least $10,000. After a few days, A sent B an e-mail says the bottom line is $15,000. But B did not see it and had the card sold to C for $10,000. There is not consent between A and C. The contract is not enforceable. C may sue A for damages or recession."

In addition, is the agent liable in this case?