Oh, the woes of buying a second-hand Vocaloid. I tried installing and activating my physical copy of Kaito V3, only to be told that his serial number had already been used too many times :(
I've dropped the seller an email (in decent Japanese, thanks to DeepL), so we'll just see what happens. At the same time, though, I'm not sure I have high hopes either. From the looks of things, the seller looks like a second-hand music shop, so they may not even be in contact with whoever originally bought Kaito and then sold him to the shop.
I would love to get Kaito up and running, but if I can't, I hope I can at least get a refund so I can put the money towards buying his DL. We shall see.