Creating a product feed is relatively easy. There are lots of extensions that can help you to create a customized product feed, like:
There are also feed management services, like GoDataFeed that may be able to help.
The bigger question is, how the Shopify users will import the feeds? And what, if any, unique requirements will they have?
If they're using some sort of an import tool or app, like a Product Information Management (PIM) solution, that may be relatively easy for them.
If they need you to take on the responsibility of more holistically organizing the product data for them in unique ways, you may want to use a PIM yourself. There are lots of options, from open-source solutions like Pimcore to SaaS solutions like JasperPIM.
My intuition is that the Shopify users are going to need to handle at least some portion of the import themselves. They'll need to adjust pricing to reflect their chosen markup, map products to specific categories, etc. They may also not want to import your entire catalog, but rather, a select portion of it.
With that in mind, if you're looking for an automated solution to manage the whole process, you may want to look at something like:
Best of luck!
