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

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