Upon arrival, the listing agent dealing with an elderly lady realizes she would be happy receiving an offer for much less than market value. The agent decides to make a cash offer for the list price the seller chose. The agent then sells the property for market value. What duty has the agent breached?

The agent has breached Fiduciary Duty

Explanation:

Fiduciary duties are in place for estate agent to protect the interests of the seller.

For example if the seller has a higher bid price for the property, the estate agent should sell the property at the higher rate. This is his duty to protect the seller's interest.

In this case, the estate agent bought the property at a lower price and sold it at the higher market value, thus breaching his fiduciary duties.