I could see why sellers would want it, and by extension customers would want it if some really beautiful models were for sale. Ref Shapeways vs thingiverse. I'm not sure now is the right time for it though, just like iTunes would have been dead in the water before MP3 players became widespread.

I said this below and was down voted for it. I think the OP has an interesting idea. I think just the choice of language comparison is what is throwing me.

I guess it depends what aspects of iTunes. I think there's a market for selling designs for things which can be printed. Even better, a market for designs that can be customised and printed.