A valid offer has to be a specific promise and a particular demand from the person who could accept the offer. In determining whether an offer is valid, you look at it from the point of view of the offeree. From a reasonable person in the offeree’s position, would they believe that they have received an offer that they can accept? Have you received a specific promise and a particular demand for the proposal? This is an objective standard because it puts a reasonable person in the shoes of the person receiving the offer so as to determine validity. If that reasonable person would believe that it is a valid offer, then you have a legitimate offer.
According to Azcentral, there have to be certain elements in the offer to make it valid. That is, it has to be specific, and it has to be definite. For example, you have to specify the price for which you are going to purchase a good or service. You can not just say a reasonable price. That would not be sufficiently specific to constitute an offer.
In other situations, you may not be able to adequately definite the person or entity who can accept the offer. There is a typical example of magazines or other advertisements that solicit proposals to purchase their goods. The advertisement looks like it is an offer, but it is indefinite as to who can accept the offer. It is merely a solicitation for potential customers to make offers to purchase. You may want to buy the newspaper, but the nearest vendor tells you they are out of stock. In that situation, they are not accepting your offer.