i saw a trade lot that was asking for CSC. can she/he really do that? if yes then how much would i get for a Dark Matter Potion Plushie?

You can buy CSC giftcards from the Cash Shop, which is how people trade for CSC. One goes roughly for about 3mil, so you just have to estimate how much sP the item is worth, and then translate it into CSC.

alright. thank you. very helpful