Yes, it's your file. We just provide a venue to sell it.
Each file goes through a rigorous reviewing process to make sure the quality is top notch.
Yes you definitely can and we encourage you to do that!
Exclusivity is on a per file basis so yes, as long as the files are completely different, you could sell one here and one on your site.
Yes, by default you are opted-out what means you sell your files only with the Regular license. The advantages to opting is that the price is multiplied 50 (fifty) times. The disadvantages are that you'll not be able to sell your file(s) on our or other marketplaces and you will lose rights for this file(s).
As the author, it's your responsibility to make sure you're not violating anyone's copyright by uploading your file. If you're unsure if you're allowed to use an asset within your file, ask the asset's owner or a lawyer.
You determine the price of your files. We just do a brief overview of the price and we can suggest you another better price, but definitely this is your choice. Too lower prices can be rejected by staff to not depreciate the market.
Providing support is a great way to ensure repeat business from buyers, but no, it's not required unless the file is actually broken.
If a category is oversaturated, that just means the quality standards go up. You'll have to give it a unique spin or make it much nicer looking for it to be approved.