it doesn't have to be a bank, your merchant store could be a bitcoin store instead of shopify. you can sell stuff for bitcoins if you don't want to get the bank involved.
Well, it means the options are completely expanded according to the customer's wishes, and there won't be any limits, right? Understand your idea, but I think it is difficult to implement. If a customer wants to receive money through BTC, your platform will have to become an intermediary to hold the BTC. If customers want to receive money through the bank. Your platform must work with those banks' payment gateways.
Let's say John wants to sell his coins, He creates a Shopify store, johnscoinsstore.com lists his coins for sale, sell his coins and he is done.
Let's say Paul purchased some coins from John's Shopify store, Now Paul want to sell his coins, so Paul sets up a Shopify store paulscoinstore.com, lists his coins for sale, and sells his coins back to John or Jane ...