The premise of the site is that a user pays a small fee to advertise an item that they want to sell. Therefore the user is responsible for adding the "products", not the administrator. The product upload will create a product page for that item. This is a rather common framework that I'm sure you're familiar with.
My initial thought was that it would be best suited using Magento - mainly because it needs to accept payments - and the products will grow to form a catalog of categorized products. However - there is no concept of a shopping cart. A buyer does not buy the item online, or go to a checkout. They simply look at the product, and contact the seller if they like it. The buyer and seller then take it from there. For this reason, I then begin to suspect that Magento is perhaps too overkill, or just simply not the right CMS if there is on checkout procedure (other than the uploader making a payment)
So then I begin to think Wordpress....Hmmm
- User's can add content via a form process
- User's can be directed to a payment gateway
- For each product listing - a series of photographs shall be displayed, in thumbnail form Zoom capabilities/rotate on the images would be a welcome feature
In short - e-commerce CMS, or something more simple?