Best way to do this transaction -
1. Take a picture of the key and send it to the person who pays for it now, that way if they are impatient they can do what Captian said - and find a digital version of the game and use their key in it.
2. Ask the buyer for a mailing address.
3. Go to post office, have them weigh it, stamp it, and send it.
If you can't provide the buyer with the cd, then you should not be reselling it.
You could though, sell the cd-key; but 20k is nowhere worth it for a key.