let your bank know it was an unauthorised transaction?
I don't know much about net transactions but isn't the whole credit card scam thing based on if someone has your correct card details they can take whatever (up to the daily limit I guess)?
So it sounds like these guys just did in essence what happens to heaps of people that get legit scammed every day...except they did it in the pretext of a business deal, mistake or not.
Take it up with them quick smart, if no exceptable outcome, take it up with your bank? (after all, it's an 'unauthorised transaction' right??)