Let's say I want to sell a similar product, but I don't have the license to sell it. What's the process to get a license to sell products with someone else's IP?
If the person doesn't have a patent, trademark, or copyright on the thing you want to copy, then you can copy as much as you want. Current IP law in the USA is "first to file" (see: https://en.wikipedia.org/wiki/First_to_file_and_first_to_invent)
If the person has any of those things, and you want to copy them legally then you have to ask them for a licence.
If the person has any of those things, but doesn't have the money to sue you for infringement, then you might be able to copy it with legal repercussion. That wouldn't be very cool though.
