MS is very aware of these kinds of sellers, and they timebomb their releases to the tech community for just this reason. They may not even expressly tell you how it will die, but die it indeed will.
a) You could activate my key on retail copy which is exactly the same as MSDN copy
b) Microsoft allows developers to use activated copy forever
timebomb? Where the hell is this comes from? Can you tell me a way that allows Microsoft to "timebomb" my key without monitor what you do and break all privacy law?
A) Obviously the key could activate ANY copy of the software, and in so doing it keys exactly what license is being used. All of the release versions are the same... the key is what sets the permission levels, and creates the triggers by which ET phones home and checks to verify that it is valid on a license count basis, and a time basis. Of course if you had any kind of legitimate ownership of this license you would already know this, and you would understand the terms of the agreement that you electronically signed with Microsoft that specifically forbid you from doing exactly what you are doing in offering multiple copies of the license for sale.
And no, I will not explain the intricacies of how time crippling works after a license check. It would be a violation of a couple of NDA's, and the effort would be wasted on an unrepentant thief like you.
B) Microsoft allows this courtesy to developers who use the software in development, NOT to little cheats who sell the license repeatedly in violation of their terms.
You, Sir, are not Microsoft, and do not have the legal grounds to sell these licenses. You are selling them illegally, you are offering tainted goods which will have no support or follow-through when they invariably fail, and anyone who buys a copy of your illegal license will be left holding the bag looking for an additional illegal way of activating or continuing to use their software when their machine stops functioning.