Considering normally a substantial agreement should account for offer and acceptance, with advertisements the general rule is that they are not offers but statements inviting further negotiations or invitations to treat with Partridge v Crittenden 1968 serving as an example, where no offer for sale was found in the advertisement. Similarly, in Harris v Nickerson 1873 an auctioneer’s advertisement to sell certain goods at a certain location on a specific date was held to be not a contractual offer but an invitation to treat only. In Grainger & Son v Gough 1896 price lists were distributed advertising certain goods at a specific price and these were found to be invitations to treat only considering that advertiser could have limited supplies of goods in question being unable to meet all the acceptances. In the latter case, however, it was stated obiter that if an advertiser was a manufacturer there could be an offer for sale since as a manufacturer he could have unlimited supplies.
This general rule about advertisements does not apply to unilateral offers with Carlill v Carbolic Smoke Ball Co 1893 being the leading case in which the advertisement of reward for performance
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of specific act was found to be an offer because intention to be bound was demonstrated by the deposit and certainty of language in the advertisement. Unilateral offer can be made to an identified individual (Great Northern Railway Co. v Witham 1873) or to public at large or particular class of persons (Poole, 2006, p. 78).
For an advertisement to be a unilateral offer the intention to be bound should be sufficiently clear, the example of which can be American authority of Lefkowitz v Great Minneapolis Surplus Store 1957, where the reference to the goods in question was sufficiently “clear, definite and explicit” and demonstrated an unambiguous and clear intention to be bound.
A unilateral offer can be accepted by a complete performance of requested act (see Bowerman v ABTA Ltd 1893, O’Brien v MGN Ltd 2001)(Poole, 2006, p. 44), and it should be clear what act is required (American case, John Leonard v Pepsico 1999). Performance of the requested act is seen both as an acceptance and consideration (Daulia Ltd v Four Millbank Nominees Ltd 1978) with offeror entitled to require full performance of condition (Poole, 2006, p. 79) with promise to perform the requested act being not sufficient (Carlill v Carbolic Smoke Ball Co 1893).