You are competing against Amazon at that point, I would pick an underserved niche that you are most passionate / knowledgeable about and start there. For instance, drone after market accessories like stickers, lights, carrying cases, etc. Look for some trends using Google, Pinterest, etc. It's business 101, find a need and serve it. Your best bet is to focus and go very deep, than casting a wide net because you're resources will be spread to thinly. I'd be more than happy to help you narrow your list and am available any time for a call. Good luck!

This is something you should ask your prospective customers. You should try and figure out what the problem is that you are solving for them in order to start understanding whether your idea has any merit whatsoever.

I think it's best if you start by concentrating in only one vertical. You need to find your audience, and you do that by focusing in one category.