I would like to add all domain selling companies to my system, but it is not in my hands. The company must provide me with a way of importing their domain inventory. Some companies inherently provide downloadable lists of their inventory, but some do not.
Usually when I contact them nothing happens. My request doesn't reach the right person in the company or they just ignore it. From my experience it works best if people that already have a business relationship with the company ask for integration (sellers/customers). Companies are more inclined to listen to wishes of their own sellers/customers than a person they do not know and have no relationship with. So if you want "xyz company" to be integrated, ask them about it! I know it works, because it did before.