Hi there. Im usualy not very active on forums or other things similar to forums. But just to make an example of a good business model for devs:
From a marketing experts point of view:
Make a plugin/theme everyone likes (Advertising)
Make the plugin/theme free (Advertising)
Tell them you can make plugins/themes for money (Advertising)
Take jobs, make money (Income)
As for a marketplace:
make everything what is in the marketplace free
pictures and descriptions of every plugin/theme
Links to devs/contacts where you can order a personalized theme/plugin