I know you mean well and want to warn the community, but posting names like that is a dangerous business. Will you actually *verify* that a seller/trader is problematic before adding his/her name to your list? Without verification, how will you know that it's a legitimate grievance vs. revenge or someone out to put a rival seller out of business? While truth always is a defense, you are opening yourself to libel lawsuits (even if you publish the names of people who have unethical buying, selling, and/or trading practices and genuinely deserve to be listed).
My advice: use the feedback system (both here and on eBay) and leave HONEST feedback that reflects the whole transaction (quality of the pins as well as ease of it); that's what it was intended for - and always check feedback before dealing with an unknown person. You aren't the first person to propose such a list and you won't be the last, but if you'd stop and think, you'd realize why the list hasn't been created (or if it has, why it isn't more common).