Why need this if you can use winrar? Winrar encrypts and secure files greatly and the fun fact is you hold the password. No need to pay even a cent.
Your idea is good for one or two persons you already know, but what about sharing it over the internet ? How will you manage to get your payment ? Here come sych innovative service ... if someone pay to decrypt your archive, you will receive instantly your payment.
Thank you for the clarification. Then this is a good idea, especially if you're dealing with a person you don't want to have a connection with but you would want to give services for payment. This is great for securing that the transaction will happen for both parties.