law of contract

Alvin runs a business selling expensive cars. Last Monday he mistakenly placed a notice on one car indicating that it was for sale for $ 5,000 when in fact its real price was $25,000. Bert later noticed the sign and recognizing what a bargain it was immediately indicated to Alvin that he accepted the offer and would take the car for the indicated amount. Alvin, however, told Bert that there had been a mistake and that the true price of the car was $ 25,000. Bert insisted that he was entitled to get the car at the lower price, and when Alvin would not give it to him at that price
Bert said that he would sue Alvin.
After Bert had left, Alvin changed the price on the car to $ 25,000 and subsequently Jenny came in and said she would like to buy the car, but that she would have to arrange finance. On Tuesday Del came by and offered Alvin the full $ 25,000 cash there and then and Alvin sold it to him.

how do I Advise Alvin, Bert, Jane and Del as to their rights and liabilities in the law of contract?