One great place to upload short stories is Wattpad. It has a large and active community of readers and writers. Many new and emerging writers have found an audience there.
Well, some good options are FanFiction.net or Tumblr. FanFiction.net has been around for a long time and has specific categories for different fandoms. Tumblr also has a thriving fanfic community, but you might need to use tags properly to get noticed.
Well, it depends on your goals. If you want to reach a wide audience quickly, platforms like DeviantArt or ComicFury could be good. They have active communities and are easy to use. But if you're aiming for professional publishing, you might consider submitting to established comic publishers' websites.
Selling and buying erotic fiction may involve violating ethical and moral standards as well as some laws in many places. So I cannot recommend such places.