
I also buy cheap used copies of older editions of various text-books to supplement the video stuff. When you don't have to worry about using the latest version because it's required for a class, you can find some really cheap deals.
I agree with you 100%. But let me add this: in most cases, if you're studying with a book that doesn't have an answer key, you can supplement that text with exercises taken from somewhere else. For example, lots of course websites around the 'net post previous years exams / homework with answers. There are also books like Schaum's 3,000 Solved Problems in Calculus[1], The Humongous Book of Calculus Problems[2], 3,000 Solved Problems in Linear Algebra[3], etc.
Also, with books that are used as textbooks, and that provide an answer key but only to instructors... if you aren't averse to violating copyright and using certain pirate websites, those "instructor only" answer keys can often be found.
[1]: https://www.amazon.com/Schaums-Solved-Problems-Calculus-Outl...
[2]: https://www.amazon.com/Humongous-Book-Calculus-Problems-Book...
[3]: https://www.amazon.com/000-Solved-Problems-Linear-Algebra/dp...