Found an [Item=Upset Marshmallow] in my vault, none in shops but many in galleries

So cute! Curious about this too!

It seems to be decently rare, but there are a couple people with multiples in galleries

You could always put it in trades and ask for offers, or throw it in your shop at 100mil.

I think a more realistic price would be something like 25-75mil, due to high risk for rerelease.