Does your application:
Allow the seller to add in different costs for freight options?
Does the seller get a notification of who has purchased what item?
Does the seller get delivery address details for each item (like and admin would do in the back office for commerce)?
How does the seller get paid?
Does the seller use their own payment gateway details or that of the main saite - if the sites, how are payment fees recovered by the admin?
Can the seller sell multiple items to different members, without the advert disappearing
Does the advert hide automatically when the product sells out or the advert expires
How can the admin charge for members using this system and how does the admin get paid?