I am plagued with this challenge too.
To date, I have opted for the approach mentioned by Mark: private galleries are unlisted, and public galleries shine in all their glory.
I would amend that suggestion with the following:
I have put all personal galleries in a specific category; it removes one level from organizational methods (I only have sub-categories & galleries to work with), but keeps everything much cleaner to know what is considered personal and which is public.
It was suggested to me that as a working pro, it may be necessary to eventually draw a line in the sand and keep one SmugMug site specifically for public/professional images, and just to pick up a second SmugMug account (likely the 'Basic' or 'Power' user). I like that suggestion, but I'm already two-feet-in to the deep end of organizing via listed (pro) and unlisted (personal) galleries.
You might also be able to keep all galleries private and add keywords to your 'sellers' and choose to use a smart gallery to draw all sellers into a publicly listed gallery (or set of galleries).
Good luck with your sales!