Business law – sale of goods contract

Bobby and Donna Jean are discussing the possible sale of a vintage  guitar owned by Bobby that Donna Jean really wants to purchase. Bobby has no interest in actually selling the guitar to Donna Jean, but decides that it would be a funny joke to pretend like he actually offering her the guitar. At one point Bobby says he’ll sell Donna Jean the guitar for $1,000. Donna Jean immediately accepts. Bobby then says that he won’t sell her the guitar because he was just joking and never intended to go through it. Donna Jean sues for breach of contract. Is there a valid contract between Bobby and Donna Jean? Why or why not (be sure to reference relevant case law)?
Phil offers to sell his house to Jerry for $100,000 but does not stipulate how acceptance is to be made. Jerry sits down and writes a letter to Phil saying “I accept your offer and agree to purchase your house for $100,00.” he then then strolls down to the mailbox and sends the letter to Phil. Later that afternoon, after the letter was dropped into the mailbox, Phil calls Jerry and says he wants to revoke the offer. Have Phil and Jerry formed an enforceable contract. Why or why not?