You mean like these?
https://page.auctions.yahoo.co.jp/jp/auction/j648801547
https://page.auctions.yahoo.co.jp/jp/auction/r403441998
https://page.auctions.yahoo.co.jp/jp/auction/q368291055
Well let's just have a look around and see what else this seller--
https://page.auctions.yahoo.co.jp/jp/auction/p765698687
What's this?
Dozens of X-Japan demo tapes? My nonsense meter is sky-high right now. The guy does at least put a "I don't know if these are real" (i.e. These Are Not Real) disclaimer on those, which he doesn't on the Malice Mizer-related ones.
Let's take that Girl'e tape as an example. The tape itself is a Denon RS-20, which seems like kind of an obscure make,
but you can just buy new blanks right off of Yahoo:
https://page.auctions.yahoo.co.jp/jp/auction/u64608923
If you look at picture 2, there seems to be a label on the back side (because you can't see through to the art)
But then when they take it out of the case and photograph both sides (check the screws to verify) there's no label. That kind of makes me suspect that this guy has a real original and a fake that he made from it - my guess is that he bought the Flower Records copy which was also white.
That copy was $600, so someone selling it starting at $50 also raises suspicion.
EDIT: actually, maybe that "label" is just the lyrics sheet stuffed behind the tape.
Of the two copies of that tape I have, one is similar to this one, unlabelled on a Denon tape with a pink cover. The other, which seems more authentic, is on an LX tape, is labelled "GIRL'E" on the tape, and has a red cover. Neither had the lyrics sheet.
However: the "faker" one in this case actually had better audio! Maybe it was copied from an original that had been played less? Or maybe both are fakes of different quality?
At any rate, I don't particularly trust this. I'd only buy from a seller like this if you're trying to get your hands on an unripped song or want to gamble on getting better-quality audio of a song that's already out there with lacking quality.
The other two Malice Mizer tapes, the type of tape they're on at least matches my copies. But there's no proof that mine are real either.