UK-based answer here:

The crux of your question revolves around whether the buyer(B) had committed an illegal act by **withholding information** that would have prevented the seller(A) from selling the good at the price he did. The act that B would seemingly be guilty of would be *fraudulent misrepresentation*

A misrepresentation is a *false statement* of fact or law which *induces the representee to enter a contract*.

The important thing here is if there is a "false statement".

In your scenario, there was **no false statement made**, let alone one which induced A into selling his stamp to B.

So there is no fraudulent misrepresentation, or misrepresentation of any kind.

Looking at the law of fraud:

s3 Fraud Act 2006: A person commits fraud by failing to disclose information when =>

The defendant: 

1. failed to disclose information to another person 
2. when he was under a legal duty to disclose that information 
3. dishonestly intending, by that failure, to make a gain or cause a loss.

With regards to the scenario you've given, no fraud would have been committed because the buyer was under no legal duty to disclose such information