As far as selling watch faces goes, on the pebble watch most of the thousands of watch faces are free.
There was even a "Watch Face Generator" app that allows you to design your own watch face - within certain criteria. You have a lot more freedom than is currently given by apple, but there are still limits. I've used it several times to create watch faces that suit me. One of them has a large Apple logo in the center of a digital face with the date above it and the time in very large numbers below it.
I used it for a long to convince people that I was a tester for a Beta version of the forth coming Apple watch. I had fun until Apple made the announcement of the watch and showed what it was going to look like.
This was a share ware app designed by a developer. I doubt that Apple will have anything like it, but you never know. Right now Apple will not let anyone else mess with the watch faces, but in 3 months or more who knows.