I've had few customers wanting online stores so I just charge an hourly fee to setup their merchant account for them. That way money from each sale goes directly to them, they deal with that, and I get paid and never have to deal with it again.
But it sounds like you want to allow your customers to set up an online store where you handle all of the payment transactions on their behalf so they don't need their own merchant account. If you did this with Authorize.Net (assuming they allow it after you explain this scenario to them, and good luck with that because it probably violates their terms of service) then all of the money for each captured transaction will go into your bank account, then from there you'll need to somehow send the money to your customers minus your fee. How will you do that? Automatically means you'd have to use an ACH/EFT provider to do wire transfers into your customers bank accounts. I hope you plan on having a lot of fraud insurance.
You can charge per transaction fees without needing to be the recipient of record for the transaction. But if you want to get involved in the moneyflow at the bank level, you have to jump through the hoops for being a regulated financial services company.
In my experience, if you are a software developer, it's probably easier and more profitable to be a reseller/affiliate of a financial institution.
But it sounds like you want to allow your customers to set up an online store where you handle all of the payment transactions on their behalf so they don't need their own merchant account. If you did this with Authorize.Net (assuming they allow it after you explain this scenario to them, and good luck with that because it probably violates their terms of service) then all of the money for each captured transaction will go into your bank account, then from there you'll need to somehow send the money to your customers minus your fee. How will you do that? Automatically means you'd have to use an ACH/EFT provider to do wire transfers into your customers bank accounts. I hope you plan on having a lot of fraud insurance.