Black Spirit Claws are rare drops in a very limited number of zones, and expensive to buy if more than one outfit needs slots. For myself, I have several outfits I swap between as the mood strikes me on multiple characters, and the prospect of applying a crystal slot to every outfit is just not worth thinking about. 

Most people don't swap underwear like they do outfits, so moving the unlock over to those instead would be a nice QOL change. 

Its always on marketplace.

Bro what are you on? Around 1200 claw pieces sell daily and 45m is nothing for a full claw

Sure, 45m is nothing. But did you read the part where I said "multiple outfits"? That 45 mil can quickly become 450m. I have 7 outfits just on my witch, same on Shai, and another 35 or so across the rest of my characters. Nice QOL would be switching it to underwear instead, so folks only need to use one per character instead of per outfit. 

As for sales, several days ago there were none listed and a bunch of pre-orders. I didn't check it today, but it's not like the market is just overflowing with them. 

I have almost every outfit for my main, many outfits on other characters and I'm broke. Maybe buying a claw for each outfit didn't help but it's not terrible. It's about the same as a trip to LoML and back LOL