Final answer:
The person with the legal authority to sign the name of the seller to a contract of sale would be an attorney-in-fact, who has been granted this power through a power of attorney.
Step-by-step explanation:
The individual who would have the legal authority to sign the name of the seller to a contract of sale is identified as an attorney-in-fact. This person has been granted the power to act on behalf of the seller through a legal instrument known as a power of attorney. The power of attorney allows the attorney-in-fact to make binding decisions, such as signing legal documents, in accordance with the authority given in the document.
A special agent may have limited authority for a specific task but not necessarily the signing of contracts. A listing broker typically acts on behalf of the seller in real estate transactions but does not have the authority to sign contracts unless given that specific power. An attorney at law could also serve as an attorney-in-fact if they are designated to do so, but merely being an attorney does not inherently give them the power to sign contracts on behalf of a seller.