Liability of Agent to third parties

• Name of principal disclosed
• Agent is not liable except:
• Where the agent contracts outside the scope of his actual or apparent authority;
• Agent agrees to be liable;
• Usage or custom makes the agent liable;
• Agent contracts by deed in his own name; and
• Where the principal is in fact non-existent.
• Kelner v Baxter (1866): where promoters of a company signed"on behalf of the proposed company" for goods.
• Black v Smallwood (1966): where the directors signed as directors on behalf of the company.

Among them, I hope to explain the meaning of "Usage or custom makes the agent liable;" and give examples.