How widespread is sharing of slides/assignments among educators?
It is common for college professors to use teaching materials from outside sources. There is generally no an expectation of originality when it comes to teaching materials.


  Do college professors commonly purchase their lesson plans and assignments from vendors? 


Many textbooks come with a set of slides, distributed by either the publisher or the author of the textbook (often for free). Instructors may choose to use these exactly as they are, or adapt them. Some books come with other materials too, including assignments.

Personally, I often don't like the textbook authors' prepared materials and prefer to develop my own. But that's my preference.


  Do instructors and universities share premade courses with each other?


Yes, this is also common. I have shared my course materials with instructors at my own university and at other universities. Many of my course materials are hosted online with an explicit Creative Commons license allowing others to use them. 

I have also participated in projects that received NSF funding specifically to develop course materials and other resources to share with other instructors.