Time and time again you will be brought back to the fact that the fundamental basis of contract law is the agreement of the contracting parties.
The thing that distinguishes the law of contract from other branches of law is that it does not lay down a number of rights and duties which the law will enforce. In other words the law of contract does not lay down a list of things that are either legal or illegal, or things that must or must not be included in a contract.
The law of contract lays down a general framework of principles, subject to which the parties may create rights and duties for themselves which the law will uphold.
The parties to a contract, in a sense, make the law for themselves: so long as they do not infringe some legal prohibition, they can make what rules they like in respect of the subject-matter of their agreement, and the law will give effect to their decisions.
For example if June says to Fred that he can buy her pen for £100 and he agrees there will be a legally enforceable contract between June and Fred. The fact that Fred may have been foolish in agreeing to buy a pen that is only worth £1 is irrelevant - the parties to the contract have agreed to the price of £100.
Whereas it is generally true that agreement is reached when one person accepts the offer of the other person - in other words there is an actual agreement between the two persons - there are two qualifications:
firstly the objective appearance; and
secondly terms implied by law.