Is it possible to transfer a Diablo 3 key to another Battle.net account?

Solution 1:

A Diablo 3 License is Non-resellable. Once the key is activated it is tied to the owner's Battle.Net account, which is non-transferrable and cannot be sold under any circumstances at this time, per the term of Blizzard's EULA.

So NO, your friend cannot sell you his copy of Diablo III in any legal way.

Solution 2:

I was able to once get them to release a cd-key for me. I had to call Tech Support and they were willing to release my Starcraft II key for me. The only difference is I told them I was going to put a new cd-key in for the Limited Edition Starcraft II. So your best shot is to call Tech Support and try it that way. He will have to call. Just say you had a second account and you wanted it on that one instead. I am not sure if that would work but that is your only legit shot of doing it.