How to decide the pricing of a C2C business app?

by Haji

I have been working on iweydii app, an app where you can buy and sell used items by chatting with sellers, who simply have to take a picture to list the product in the app.

We wanted to price the list items as packages for example. $10 for 12 posts in the app. Also when a user posts the item they can boost the item to be on top of the page which is another package $1.99 a day. How do we convince sellers to pay to list their items? Or should we ask to post the item for free and later focus on selling him to boost the item? Thanks