Users can add separate products in admin. They can edit , copy , view , delete only their products . You can set up for which user group this will work. All others groups can work with products normally.
By user i mean default user system in opencart
Help with installation is free but you must send me admin access to your site.
All products and orders from all vendors are visible for admin
Vendor user can see only his products and orders.
Protected for any case if user try to access other products by changing post or get ( hack )
for admin use username admin password admin
Go to admin->System->User Group , select group that will have separate products and press Edit , and for Separate products(vendor) select Yes.