Analyze all potential liability and defenses

Scenario:

Debbie Dealer operates an antique shop. While traveling, she buys a Union cavalry officer's handgun for $1,500 from Seller. Dealer takes several photos of the handgun and Seller agrees to ship it to Dealer's shop. When Dealer arrives home, she immediately shows the photos of the handgun to Bob Buyer. The parties shake hands on a deal to sell the handgun to Buyer for $2,000, payment upon delivery.

The next day, Buyer regrets agreeing to the deal without first having an opportunity to actually examine the handgun. Buyer tells Dealer that he will not pay the $2,000 unless she first allows him to have the handgun examined by an expert appraiser. Dealer becomes angry and tells Buyer, "A deal's a deal. I'll expect my money when the handgun is delivered to you."

When the handgun arrives at Dealer's shop, she does some internet research and discovers that the handgun was issued to a general who played a prominent role at the Battle of Gettysburg, which increases the value of the handgun by a factor of ten. The next day, Dealer receives a letter from Buyer stating, "Sorry. You're right. A deal's a deal." The envelope contains a check for $2,000. Dealer sends the check back to Buyer with a note stating, "Buyer: Because you backed out of our deal, I will not sell you the handgun. //Signed// Dealer."

A few weeks later, Buyer learns that Dealer is offering the handgun for sale at her shop for $20,000 because of its connection to the Civil War general. Buyer brings suit against Dealer for breach of contract.

Is Buyer likely to prevail against Dealer in his suit for breach of contract?