Kaplan Lecture Notes does not have International Standard Book Number (ISBN)
because they are printed privately and they are not meant for publication. They are just like hand-outs distributed to Kaplan Live course
students and they are not for resale.
The reason you see them in Amazon
because Kaplan sells those notes that are "overstock" in their centers.
By the way there's no other online or offline library that legally sell KLN other than Amazon
Hope that helps.