A few thoughts:

1. As long as there's an internet, there will be people redistributing other people's work, GPL or not. It's the price of doing business.

2. There will always be a theme that's the equivalent of a Gremlin for a lower price. But people who appreciate the value of engineering will pay more for a theme that's more of a high performance vehicle (that gets you there in style).

3. Authors won't be supporting themes sold through the black market. That was one of my biggest costs when I sold themes. So, hey, saving me support costs for people who are too cheap to shell out isn't necessarily a bad thing. They'd probably be more likely to be high-maintenance support customers, too.

4. Themes from the original seller won't have PHP-VD (malicious code). You can't guarantee that in themes purchased on the black market.