Advertisement is a unilateral offer
Spike (appellant) needs advice as to his rights against Crab-Apple (respondent),that the advertisement is an unilateral offer, appellant has accept the offer by his behaviour and respondent has revoke the offer wrongly before completing respondent acceptance.
The appellant can argue that the advertisement is a unilateral offer and respondent published it having an intention of a binding agreement. In the advertisement, the respondent has included the exact time, date and conditions. People can clearly identify by looking at the advertisement that the respondent had a serious intention of having a binding agreement.
A unilateral offer can occur where one party, the offeror, promise to pay for the performance of another, that is, a conditional promise. Promises in unilateral offer can perform in many ways. The acceptance of the unilateral offer takes place when the offeree performs the act in specific way. If offeree has performed the act offeror cannot reject it. For example, in the case of Carlill v Carbolic Smoke Ball Co  and O’Brien v MGN Ltd  .
In Carlill case they have advertised that they would pay £100 to any person who will get influenza after using the smoke ball specified period. As in Crab-Apple advertisement; they would give the Apple-Pip for the first five customers for £50. Both these cases are equal as a unilateral offer made to the entire world.
In Carlill case Bowen LJ stated: “……. Although the offer is made to the world, the contract is made with that limited portion of the public who come forward and perform the condition on the faith of the advertisement."  Likewise appellant started performing having faith in the advertisement. Any reasonable man would act on Crab-Apple advertisement.
However, the respondent may well argue that this advertisement is an invitation to treat not a unilateral offer. If the advertisement is an invitation to treat, only the respondent can accept it. They may use the cases like Partridge v Crittenden  to illustrate invitation to treat. But in this case respondent have made an advertisement & put up on the newspaper. According to the above facts we can clearly recognize in advertisement what the respondent made is a unilateral offer not an invitation to treat.
According to the appellant argument where" he has shown his intention to accept the offer by his behavior" it can be summed up by Errington v Errington and wood  .In Errington case lord Denning has stated: “The father promise was a unilateral contract – a promise of a house in return for their act of paying the installments. It could not be revoked by him once the couple entered a performance of the act, but it would cease to bind him if they left it incomplete and unperformed, which they have not done. They have acted on the promise and neither the father nor his widow, his successor in title, can eject them in disregard of it"  .
As Denning LJ stated appellant had an intention of accepting the offer by queuing up to be the one of the first five customers to buy Apple-Pip and defendant cannot revoke the offer because appellant was performing on respondent promise by queuing up.
However respondent may argue that if this is a unilateral offer it is freely revocable until full performance by offeree. They will argue using the case Daulia v Four Millbank Nominees Ltd  where respondent revokes the offer in last minutes. But in this case (Spike v Crab-Apple) appellant has accepted the offer by the behaviour before revoking the offer or changing the offer. After offeree has accepted the offer; offeror cannot revoke the offer. As mentioned above in the case Errington and what Denning LJ has stated; it cannot revoke once appellant has started performing the act. If it is only incomplete offeror can revoke the offer.
The main strong point for the appellant is the respondent has revoked the offer wrongly by only making an announcement before 30 minutes opening the store. Revocations must be communicated and offers made to public by newspapers should be revoking in the same manner. It is better if it can revoke in the same day.
In Shuey v US  defendant won because the revocation was published in the same manner though appellant did not know about the revocation. Strong J has stated: “…….but by means of a published proclamation, he should have know that it could be revoked in the manner in which it was made"  .
Respondent must know a reasonable man will only look in the newspaper where the advertisement was published to get information about the offer. As Strong J stated respondent has wrongly withdrawn the unilateral offer.
Finally, after considering all the arguments mentioned above it can conclude respondent had an intention of having a binding agreement and the advertisement is a unilateral offer. Respondent cannot say it is a “mere puff" according to the above arguments and cases illustrated. If we apply objective test here; any reasonable man would perform according to the advertisement to buy the Apple-Pip just for £50. Therefore, the courts may conclude that valid acceptance occurred when appellant started to queuing up. It future proves that defendant has wrongly withdrawn and the revocation is not valid according to the above arguments. Looking at all the agreements and facts it is clear that appellant is entitled to buy Apple-Pip for the price of £50, which is the subject matter of the unilateral offer.