"...I don't understand why I have to get a license for the product I made by hand. I sell this product on many e-commerce sites. Is this a problem on Etsy?"
It is a problem everywhere you are selling it, not just on Etsy. It doesn't matter that you made it with your own hands. The idea of that character belongs to someone else, so you can't use it without paying the owner a fee.
Even if you consider it fan art, you are allowed to make fan art for your own use or give it away for free. But the minute you charge one penny for it, it becomes infringement, and there are laws that address that.
Spiderman in the comic books is owned by Marvel Comics, now a subsidary of Disney; Spiderman on the big screens is owned by Sony Pictures; Spiderman in form of merchandise is owned by Disney as well.
Copyright and Trademark are LAWS, not just Etsy rules.
If the copyright owner sues you, you can receive up to a $150,000 fine and/or a year in the county jail.
If a trademark owner sues you, you can be fined up to $250,000 and/or sent to Federal Prison for five years.
Disney has sued a whole bunch of Etsy sellers in the past, and won. They have a whole team of world class lawyers, and even with a poor lawyer, you would be found guilty, because that is not your intellectual property, and you have not paid the owner to license it, which costs many thousands of dollars.
Disney and many other big trade names check Etsy regularly looking for people infringing on their products. They don't always give you the opportunity to remove it. Some just go straight to the lawsuit stage.
Just because a lot of other people are doing it is no excuse for you doing it, too. It won't make you any less guilty. They just haven't been caught, yet.
