Chip Bennett has done this already with his theme Oenology. All of Justin Tadlock's themes are well documented and commented as to serve as a reference also. All of which are free.

I recall Sarah Gooding wrote an article on WPMU about a Naked WordPress theme that was well documented, she also wrote here about how WordPress itself is to be given inline documentation treatment.

There maybe room for a commercial theme, but after Justin's experiment at Themeforest I would think the market isn't overtly concerned with best practices. But nothing ventured, nothing gained. If they can foster an active friendly community with good a learning / support environment people would have no problem paying.

Looking forward to see how this venture goes and wish them all the very best.