But if I give you the letters in exchange of the money, this doesn't make it any less blackmail...
And having extra MacGuffin's to sell shouldn't change it, either.
You could have made copies of the letters.
And after selling you one, I will still have one less MacGuffin. Presumably these can't be copied easily. And if they can, e.g. if it is a digital file, then what is it that you really want to get? Access to the information in the file or preventing access to the information for others?
This made me think of some related concepts: NDAs and exclusive rights contracts.
For a more parable-ic version of this, see here.
Suppose I make a precommitment P to take action X unless you take action Y. Action X is not in my interest: I wouldn't do it if I knew you'd never take action Y. You would want me to not precommit to P.
Is this blackmail? Suppose we've been having a steamy affair together, and I have the letters to prove it. It would be bad for both of these if they were published. Then X={Publish the letters} and Y={You pay me money} is textbook blackmail.
But suppose I own a MacGuffin that you want (I value it at £9). If X={Reject any offer} and Y={You offer more than £10}, is this still blackmail? Formally, it looks the same.
What about if I bought the MacGuffin for £500 and you value it at £1000? This makes no difference to the formal structure of the scenario. Then my behaviour feels utterly reasonable, rather than vicious and blackmail-ly.
What is the meaningful difference between the two scenarios? I can't really formalise it.