An invitation to negotiate is not an offer.  An invitation to negotiate is merely a preliminary discussion or an invitation by one party to the other to negotiate or make an offer.  For example, a school asking a teacher whether or not the teacher wishes to continue teaching at the school the following year is a preliminary discussion and not an offer that could be accepted.  If the teacher indicated that he would like to continue teaching, then a formal offer could be made.

Generally, newspaper advertisements, price quotations, and catalog prices are just invitations to negotiate and cannot be accepted in a contractually-binding manner.  No seller has an unlimited supply of any commodity and therefore cannot possibly be deemed to have intended to make a contract with everyone who sees the advertisement or seeks to accept the price offered.

Assuming that an offer is made with intent to be bound, it still is not a legal offer unless it is communicated to the offeree.  This offer may be communicated by the offeror or at his direction.  If the offeree hears about the offer indirectly, through the grapevine so to speak, he cannot accept the offer until it is communicated to him by the offeror or at the offeror’s direction.

An offer can be withdrawn before acceptance and therefore prevent a contract from arising.  If an offer is terminated, an attempted acceptance after the termination has no legal effect.  Ordinarily, an offer may be revoked at any time by the offeror.  All that is required is the showing by the offeror of his intent to revoke the offer and communication of this intent to the offeree.