David advertises a bicycle for sale on Facebook. Sally reads the advertisement and telephones David. Which of the following scenarios is correct?

Sally says to David 'I accept. I will be coming to collect the bike this afternoon.' This constitutes and agreement.

Sally says to David 'You want too much for the bicycle in your advertisement. I will give you 50% of what you want. See you this afternoon. What Sally has said to David constitutes acceptance of an offer.

Sally says to David 'I would like to buy your bicycle for 50% of the price you have advertised.' David says 'OK see you this afternoon.' This constitutes an agreement.

Sally says to David 'I would like to buy your bicycle for the price stated.' David says 'Sorry, I already sold it to another person.' This constitutes an example of equitable estoppel.