Bulk T-Shirt Designer for commercial Use (Merch by Amazon etc.)

Hey,
Im looking for a longterm T Shirt Graphic Designer. If we fit together and I like your Job, I will come back to you often.
My Questions:
- can I use your Designs 100% to sell them ? Not only in the US, also in Europe ?
- Do you sell each design more then 1 time ?
- Is it possible that I will give you an Idea (for example: I want 20 different Designs, which have to do with "Feminism" or "Gaming" or... ?)

I will pay you 1,5$ - 2,5$ for each Design.

Thanks for the answer :)