Beatrice owned a car which she wanted to sell for $5000. She advertised the car for sale by placing information brochures at the local supermarket notice board, to target potential buyers in her neighborhood. Upon reading the advertisement on the board, Leonard telephoned Beatrice and said that he would buy the car for $ 4,000. Beatrice said that she had changed her mind and was not sure that she would sell the car, but that if she did she would want at least $6,000. After further discussion Leonard offered Beatrice $5,700 for the car. 

Beatrice refused to sell for that price but said she would keep her offer open for five days. The next day Beatrice met an old friend, Patricia, who said she was looking for a car. After further discussion Beatrice sold her car to Patricia for $6,000. Later that day Leonard overheard Patricia boasting about the car she had purchased from Beatrice. Leonard immediately rang Beatrice and told her that he agrees to pay $6,000 for the car. Beatrice replied, "I'm so sorry, Leonard, but I have already sold the car to Patricia”. The brochure made no reference to price. 

Explain to Leonard all the aspects that would be necessary to establish agreement in these circumstances.