OK, from what I can understand from the situation, it seems that itch.io is having cash flow problem
This is not true at all. This topic has been open for quite a while, and so the developers who have posted may have payouts delayed for various reasons. Many people filled out the tax form incorrectly and we've canceled pending payouts from developers so they could confirm their information about re-open the payout request. There seems to be some confusion about this though.
I do think your idea of having a seller fee be a requirement for someone to buy a game. It's unlikely we'll do something like this though. The amount of money it would be insignificant compared to the revenue generated from regular sales.