问大家一道关于合同法的问题，谢了！

Alan decided to sell his car. On 1 May, he placed an advertisement in the Daily Moon which stated:

“VW GTI 2000 model. One previous owner. Serious enquiries from people with ￡3000 to spend. First come, first serve.”

Brian saw the advertisement and telephoned Alan asking Alan whether he will take  ￡2750 cash for the car. Charles also saw the advertisement and telephoned Alan saying that he will buy the car for￡2800. Alan eventually offered to sell the car to Charles for￡4900 and Charles said that he would need to drive the car before he could agree such a price. They agreed a time to meet at Alan’s home at the weekend for a test-drive. When Charles turned up at Alan’s house at the weekend for the test-drive, Alan said that he had changed his mind about selling the car
In the meantime, Brian not having heard from Alan, telephoned Alan again saying that he will buy the car for ￡3000 but Alan said that car was no longer for sale.

QUSETION:
Advise Alan of any potential liability he may incur to Brian and Charles for breach of contract.
怎样全面的分析这道题？