Originally the law in consumers was developed from two main areas, Torts and Contract law.
Torts was developed to remedy 'wrongs' that people committed. The fundamental question in torts is "Should the loss lie where it fell ." So should B who was negligent to A, pay for A's damages? To determine this we have legal tests, such as Duty of Care, Breach of Duty and Damage.
Torts is difficult to define. There are heaps, including Trespass, Negligence, Malicious Prosecution, Intimidation, Breach of Statutory Duty, Injurious Falsehood. The most common in consumers are: Negligence and Deciet (to do with passing false statements that the speaker knows are false - also called "Fraudulent Misrepresentation")
Torts was developed to stop people from attacking each other, instead to seek a remedy in the courts, predominantely this was applicable in cases of battery and assault.
Contract law however was developed to enforce promises people make. An important document is the Statute of Fraud (1699), which was repealed under the Colonial Law Revision Act and replaced with our laws now.
Things to consider include the reason for signing, to ensure that a person understands terms, for identity and the like. There were certain historical factors to consider that are still applicable now in the signing of documents.
Contract law under common law can be a little unfair. For example, if you have a mistake in your contract, the contract may be voided at Common Law (or ab initio - 'from the beginning'). So let's say you have already paid $2000 for somebody to do something and the contract is voided, you have lost your $2000 under common law. So the development of Equity intervened and it provides other remedies that order people to do things. In this case, equity would provide what is called a Quantum Meruit Order, because a person who has $2000 for a mistaken contract is unfair. Equity will intervene where common law is unfair.
My little spiel on things.