Is it possible to trade Sp or items for cash shop credits? Just want to know. :)

Yes you can :) In the trade center cash shop credits can be offered on trades. You can also buy csc cards in usershops:

100 Credit Gift Card 250 Credit Gift Card 500 Credit Gift Card

They also come in 1000, 3000, and 5000.

You can buy credit gift cards from user shops or trades (e.g. 100 Credit Gift Card) and then redeem them for CSC. That is the only way that I know of. =)

You can buy these with sp and they will give you cash shop credits:

And you can use the trade center to trade items for cash shop credits. :)

Edit: Darn, I was ninja'd, but my reply is the most colorful.