David is negotiating with Elsa for a used iPhone 8 she has advertised for sale. Elsa advertised the phone for $300. David inspected the phone and offered by telephone to buy it for $250. Elsa replied that $250 was too low and said anything below $280 was non-negotiable. David contended that the price was not reasonable and asked for more time to consider.

Thirty minutes later he phoned Elsa saying that he had changed his mind and agreed to pay $280. However, Elsa replied that the price was reverted to $300.

Advise both David and Elsa of their legal position