On April 1, Jennings, a licensed used car dealer, wrote a letter to Wheeler in which he stated:

"I have a 1955 Thunderbird convertible in mint condition that I will sell you for $15,000; I will keep this offer open for you at any time until May 9." [Signed], E. Jennings.

On April 15, having heard nothing from Wheeler, Jennings sold the Thunderbird to another party. On April 29, Wheeler told Jennings that he accepted the offer and tendered the $15,000, in cash. When Jennings told Wheeler he had already sold the car to another party, Wheeler claimed Jennings had breached their contract.

Is Jennings in breach, why or why not?