What could i print & sell

I do small freelance work for designs/prints. I recently helped my brother design a patent for use in the medical field. While he COULD have me print a ton of them and start a business off of it, it is better to show the printed part to some board in a meeting. The medical field wants plastic, yes. But If you want to get into the business side of things, instead of printing just to sell the patent (which is what I am doing), then any 3D printer is fine. My SD2 runs everything I've designed...for the most part. A prototype only has to be present and operational to sell the design. You could have profanities regarding ZZ Top scribbled all over it. But as long as the design delivers what you are promising, it doesn't have to withstand an anvil.

As far as WHAT to print for the intentions of selling, remember that there are design laws and patents on most things. You cannot produce them for the whole intention to sell while the designer makes zero. It is illegal, and more than that, shady and unethical.
The beauty of 3D printing is this: You are only limited by your own imagination.