There's another good bit of advice (although it comes from someone who doesn't sell, so keep that in mind.) Have both "artistic" and "technical" beta testers. It's good to have testers who will post gallery pics and help drum up sells, but you also need people who will brutally try to break it -- bend models in unexpected ways, flip characters upside down to see how textures blend at the feet, and so on. I think it'd be good to have both.