Well, seeing as Amazon allows publishers to publish their books for free, you have an uphill battle facing you. They also have 100 million people paying for Prime memberships as well as billions of customers using the site each year.
In reality, it is going to take at least 1,000 customers/vendors or so before anyone would ever even take interest in paying for your site. This means, you are going to have to give away memberships for free for a while, all while spending time, money, and effort on getting traffic to your site.
The issue you are facing is, you are asking people to pay you to use a site that has zero traffic… in order to sell their books where there is zero traffic. No matter how many customers you want to ever have, you will have to pay for them to get traffic on your own until you are generating revenue from paid vendors.
“But Jason, this gives them a place to process transactions and sell books without all that Amazon clutter.”
And my answer to that argument is… “Amazon has traffic that will get any competent author a couple of hundred dollars per month in revenue PER BOOK, and they haven’t paid a dime to make it happen.”
You can’t compete against traffic… or reality. If you aren’t willing to drop $10,000 to $30,000 on this project to get it started, you may as well not start. If you have the cash to burn, give it a shot.