I'd consider paying for the ability to use some icons for commercial purposes. Perhaps you could get some sort of affiliate program going? Then you could add another rights type - 'Allowed for commercial use (purchase)' or something like that. Then you could start allowing designers to offer their icons directly. A centralized place for purchased icons would be much nicer than what you have to do now - search all over the web and trudge through lots of designer's websites to try and find what you're looking for.