I'm looking for a website where on my Minecraft server, people can register accounts, and there's a template for, "Buying:" "Selling" "Trading:" that's free and easy to use?

Sorry to say but such websites often cost significant cash to setup and run.
What is the biases of the trading ?

Well, it's a Pixelmon server, and I was looking for a website where people could upload stuff as,

BUYING: Pikachu
SELLING: Chansey

SELLING: Moltres
BUYING: Mewtwo

Something to avoid chat spam.