Oh my goodness! These are so wonderful! Daniel, great work and I hope you do find success getting these made into stamp sets. This is a great style that would be a good addition to the types of stamps that are currently available!
I love the character development! With adult coloring books are all becoming the rage. I wonder if there is any way to explore that market. Most of the books I have seen are extremely detailed designs. This would be a much lighter contrast. Good luck with develping a marketing plan!