Since you have taken to the sales forum to discuss your private affairs I am compelled to chime in.
First off, from my understanding, the crystal issue was disclosed to the buyer only after he agreed to purchase the watch from you. Second... you erroneously advised the buyer that the watch was a random serial when in fact it was a K serial. It doesn't matter that you think the price is a "killer launch" price. Nor does it matter that you didn't know what "random" meant".
I don't think that there is a single member here that would dispute the fact that the crystal and series knowledge can and does impact a watch's value. The buyer paid you for the watch, and that automatically qualifies him as a straight up buyer. It seems here that you have changed the terms after the fact, so why should he not be free to do so as well?
This is not me taking sides... this is me pointing out the facts as I see them.
Also, as most members on this site use third party insurers that provide the fedex template, you should absolutely defer to the rules of the insurer. Fedex has no knowledge of the value of the package, and in fact end users are NEVER pernitted to allow them that knowledge. The only way that the package is protected is if the first contract is completed, and then a second contract is opened for the return. And fedex absolutely bills for the package return! Been there and done that.
The buyer, in my opinion is the only one at risk right now, and his rights should be protected. As well, the buyer should have every right to receive and inspect the watch and decide then and there whether or not to accept it with no further refund. In my experience where there is smoke there is fire...
In my opinion if the buyer returns the watch to you, you should make them whole. The original errors are yours and you should own them.
And never again air your dirty laundry openly on the forum. This is not the place, especially knowing full well that the buyer could not even respond to you here. You have opened up the window for people to question your integrity far more than his.