@Milan Petrovic -

You wrote "If a premium developer sells a theme and then not provide support for the themes, than it would be a good thing to have some way to ‘report’ such developer and warn other users to stay away."

That is very dangerous and against the freedom of software. Very counter-productive IMHO.

There is nothing bad with selling a GPL compatible theme and NOT providing support. GPLed software comes AS-IS. You can take money for the service to pass over the GPLed sourcode, the GPL does not hinder that. The opposite is the case, it explicitly states that taking money for GPL software is an option.

There is no need to "report" such developer or even to warn other users "to stay away". That's counter productive. If I sell a customer a GPL compatible theme I educate him about the rights he has (that's part of the license BTW). And the good point of the GPL here is that that customer can actually get support in the forums, because he can pass on the themes code to any support personnel, not only the one of my company.