How widespread is sharing of slides/assignments among educators? Often, when taking a course at the university level, I amuse myself by Googling phrases from the instructors' slides, assignments, and other materials. I have found, without exception from any instructor, that the material is more or less identical to some resource I can find online, often course materials posted by a professor at a highly prestigious university (e.g. Stanford). In some cases I will find material that is 10 - 15 years old; other times it is undated. Sometimes I can find the same material posted by a dozen different professors at a dozen different universities.

A few times in the past, I've entertained the idea of reporting the professor for possible plagiarism, but I've never followed through on it because


If the material is undated, there's no way to know who published it first
For all I know these slides and assignments can be purchased from vendors. I have occasionally had a professor use a complete course package, where the publisher of a textbook also offers the assignments and tests to accompany the textbook. 
I have been unwilling to risk having to re-take the course, if the instructor is suddenly fired mid-semester for plagiarism


(In some cases the materials are sprinkled with enough informal language that it's obvious that they were not purchased from a vendor.)

Since it seems like all of my professors get all of their slides and assignments from somewhere else, I am curious how common it is to share course materials between instructors. Do college professors commonly purchase their lesson plans and assignments from vendors? Do instructors and universities share premade courses with each other?
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.