I got one from random and saw it was the prize for the trivia but their is also none in market... how much should it be sold for?

They are glitched in the shop search. They're not worth much at all, honestly.

They are usually worth near nothing, but since the achievement came out they have grown exponentially in price. Basically any price you name is fair now, sadly. @

If you put the black candy bar in your shop and autoprice it, it's currently at 20k.

Black Candy Bar 0 sP 20000 sP (20000 sP)

this is a common item that's actually fairly cheap, but glitched at the moment. I would recommend if you come across any glitches on the site to post them in the bug forum.

however, due to the fact that this would normally be a searchable item in shops.. I'm going to lock this.