Alex sells second-hand books. He advertises in a trade journal a rare book for sale at 500€. On Monday Maria sees the ad

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Alex sells second-hand books. He advertises in a trade journal a
rare book for sale at 500€. On Monday Maria sees the advert, calls
Alex and tells him that she will buy the book for 500€. Alex
replies, “I made a mistake in fixing too low a price, but I am
prepared to let you have it for 1.000€. Think about it. I guarantee
not to sell the book to anyone else before Saturday. But if you
want it, I must receive your reply in writing by Friday.” On
Tuesday morning Alex sells the books to Martin for 1.500€. On
Tuesday afternoon Maria sends off an e-mail agreeing to buy the
book for 1.000€, but she mistakenly writes the wrong e-mail address
so that the e-mail does not reach Alex. At the same time, and as a
precaution, Maria sends by a regular (postal) letter a copy of the
e-mail which arrives at Alex’s home on Thursday morning. On
Wednesday morning, Alex calls Maria and leaves a message into
Maria’s answerphone saying that the book is no longer for sale.
Unfortunately, the answerphone is not working properly, thus the
crucial part of the message cannot be heard. On Wednesday evening,
while out with friends, Maria discovers that her friend Martin has
bought a rare book.
Define the essentials elements of a contract.
Describe what are offer and invitation to treat.
Explain the difference between these two concepts.
Advise Maria on her rights.