Trial and Error
Purpose and Industry
Art and Music
I have always had my "the only 10 toys children need" in mind as my marketing focus:
I'd already decided not to sell "sets" because I don't want to imply that there are a necessary number of anything big. I'd already decided to include patterns and instructions for making with anything (playropes are easy for older kids or parents to make themselves - it's just finger knitting) I sell.
I'm actually thinking of contacting Payne to see if I can have permission to reprint his thoughts on what is necessary instead of telling people their kids need these specific 10 toys. When I talk to people it's easy to make it clear that if you can say YES to the kids using the household linen closet and tables and chairs and couch cushions then you don't need to buy any building up toys. If you don't feel comfortable letting them use the couch or diningroom table or their bunkbeds as the basis for a fort then some sort of playstands, building cloths, clips and playropes are a good investment, but they aren't necessary by any means.
In fact I think that is the 10th element missing from his list - PERMISSION. He does talk about this, but it isn't concisely on the list. If you can give children permission to play with things already in their household environment that are not designated as toys then they need even fewer actual toys.