since i don't see a forum, blog, or newsletter from itchio about top selling assets or content
Why would you expect a newsletter of all things. Or a blog about top sellers. The few things Itch kinda promotes are in the featured sections and this tells nothing about it beign sold often.
What tells about being sold often is the actual list, if you sort it accordingly. I guess that list is a running average or something and not an all time ranking though.
https://itch.io/games/top-sellers/store
https://itch.io/game-assets/top-sellers/store
https://itch.io/comics/top-sellers/store
https://itch.io/books/top-sellers/store
If you want to research what is unpaid and people still pay for, use the free option.
https://itch.io/games/top-sellers/free
This is distorted a bit by "individually priced" files. That is, a free project where some files are behind a pay wall.