Would it be illegal to sell notes that I have made using my own reading and understanding of a textbook and nothing else (like nothing from lectures of classes (they were all online so there were no lectures)). I'm assuming it would be illegal if I were to copy images from the textbook but if my notes were snippets of the singular textbook and what I find to be important information, of which are mostly notes that I've written verbatim, would that be illegal and considered as a condensed version of the textbook? If so, if I were to cite the textbook would I then be able to profit from my notes?
Your re-explanation of the subject matter is your own literary product, which you have the exclusive right to profit from and distribute. You may copy portions of other works as part of such a work, under the fair use provisions in the US, which would allow you to copy a limited amount of the text for illustrative purposes. There is a ballpark figure of "under 10%", under the "substantiality" factor. On the other hand, you cannot freely copy "the heart of a work", in case the heart of the work is less than 10% of the whole work. It also depends on what kind of a work the original is (science textbook vs. literature).