I have definitely been asked to do just this about fifty times now. I refuse every time. There is no way I want to write the code to add new products to a shop via third party WWW code. The API calls are laborious to setup and commit, wrapping a UX around that is even more laborious. It's a no-brainer to use Shopify to leverage product building. Especially with the ability to import/export from shops, it means using a dev shop as a sandbox for vendors is possible.
Take the path of least resistance. Otherwise, get yourself some $5/hr help that has a couple of hundred hours on their hands and go crazy.