Ann's Antiques has a rare Click Clock in its shop window marked at 1000.

On Monday Beth, a keen collector of clocks, offers Ann 500 for it. Ann says she will accept 750, where upon Beth asks for three days to think about it. Ann tells Beth that she must have a reply by Wednesday at the latest.

When she gets home Beth looks up Click clocks in one of her specialist books and finds that the clock is actually worth 1500. Beth immediately writes and posts a letter to Ann agreeing to buy the clock for 750.

The following day, Tuesday, Beth meets Carol, a fellow clock enthusiast, who tells her that she has just bought the click clock displayed in the window of Ann's Antiques for 1000.

Beth's letter agreeing to buy the clock for 750 reaches Ann on Thursday after she had sold the clock to Carol.

1. Explain, with references to legal principles and relevant case laws, the legal effect of placing of the click clock in the shop window by Ann with the price tag attached.