The No Refunds policy completely undermines your idea. If users cannot negotiate or return the money after payments, it is a suitable place for fraudsters, as any file can be uploaded, and when the brother party buys this file, he finds that it is incorrect or a wrong file, and then he cannot demand money or even money. Negative rating on the seller.
In short, your paid service is more like payment API and not Sell Files Securely for Cryptocurrency.
Exactly, so why would I risk both getting scammed + paying a fee, if I am buying a file for $100 and the fee is $5, then I am risking an extra $5 for NOTHING, might just risk the $100 and deal directly with the seller/scammer.
If you charge a fee to facilitate a sale, then you should freeze the funds for a day or two or until the buyer approves the transaction, and when a transaction is disputed you need to have the right staff to look into the problem and solve it, people are are well trained and know how to decide who is in the wrong, OP apparently isn't doing any of that, besides his 50k+ sales in 5 weeks is just fishy as hell.
So given all that, this is stupid at best, scam at worst.