Okay, so I've been trading on this site for a couple of months.. and there is one thing that I have never been able to get quite right...

... how do you tell someone thanks but no thanks? I guess I'm far too polite, but I don't open a trade frivolously, but I get trade offers for stuff that I am interested in, but it's not high priority for me. Or I get trade offers for stuff I have no interest in. How do you say no to a trade? I mean, we're all just folks now, right?

You hit the cancel button and in the chat say, "Sorry but I am not looking for those cards but thanks for the offer"

WarLordUSA is spot on. Just like how you'd tell someone in person that you're not looking to trade at the moment, you can just as easily tell the person in the chat box that you're not really interested in the trade at this time.