My tendency is to flag the Rares placed for sale on MP, by an individual, when they are spamming keywords. That's because it will take the biggest bite out of their efforts, if the item is taken down. There are just too many commons to bother, without seeming like a troublemaker to LL. If the gacha is of my desired interest, of course I would care to see them.
There's another huge consideration. If you've ever played gachas, trying to get that rare item, or even an elusive common to complete a set, you know it sure does feel like gambling. Allowing people to sell their rares on MP is indistinguishable from Skin Gambling. If they sold every item at the price of a pull, it would be a lot less egregious, but allowing them to monetize the chance aspect is recognized around the world as unregulated gambling.
Exposing minors to the activity, whether from ads, access to the gachas, or even the availability of the prizes is standing on very shaky ground. A partial remedy might be to classify every aspect of gachas as Adults Only.