I'm guessing these are MSDN keys, which are illegal to sell.. Just FYI.
BTW I wouldn't recommend telling your customers "These keys last forever" .. when they can get turned off very easily.
1. If your MSDN account expires or is banned (for doing the obvious), all keys will be banned.
2. If Microsoft detects you distributing keys illegally (such as selling them in this scenario), and bans each individual key.
Just some food for thought.