The first necessary element of a valid contract is that all involved parties agree to all of the details of the contract - Mutual agreement. This is sometimes referred to as a "meeting of the minds." A contract does not exist unless a sincere offer has been made by one party and accepted by the other. "I'll have some french fries," constitutes an offer. When the waiter brings the food, the offer has been accepted. An offer, however must be definite and serious. Even though a frustrated student may say "sell this piece of junk for a dollar" when his motorcycle refuses to start, he probably does not intend it to be a serious offer. IF a nearby student says "I'll buy it!" the surprised owner doesn't have to sell it. Remaining silent does not mean that one accepts the offer. A sales person's statement to you that "If you do not nodify me by tomorrow , I'll assume you you've accepted," has no Legal value. The other party must definitely state that they are willing to purchase the item for the contract to be inforceable.