How to ADD products for Marketplace Sellers?

The only form of product entry for Marketplace sellers is through the node create form create content -> product. Your case sounds rather custom and I am not sure how I would implement in a module that will assist everyone.

My suggestion: create a custom module.

Hook_perm: To add a special permission for marketplace users accessing the common items

Hook_access (with the "edit" $op): So that when a user is editing, make sure they have the given permission. You might want to create a new role for this purpose, along with a registration form that adds the role when completed.

Hook_form_alter: add to the form a checkbox. The checkbox can just be something like "Allow marketplace users to sell this product?" This drives the above two permissions aspects.

- within Hook_form_alter you can create a select list that is driven by your list of Common Items. Or you can perhaps tie the list into a View which provides links, such as "Create new [Common Item #1]" which will bring them to the node edit form.

Along that same line, perhaps looking into Product Classes will help you out. You can create a new product class for each common item. But this will only be valid if your situation is similar to "A Common Item is a TYPE of product that a user can sell.. not a single product." I hope this makes sense. Good luck!