Wacky Bucks just appear in the episode One Coarse Meal. In this Episode, it is revealed that Mr. Krabs pays SpongeBob with Wacky Bucks. In that episode, SpongeBob tells her to buy something with them, but Pearl notices that the money isn't real money.
Wacky Bucks look like US-Dollar. The difference is, that Mr. Krabs is on it as President and that Wacky Bucks is written on it. In smaller text it says "Not Legal Tender".