Unless it's a subject where it's open book and you can bring in those notes with you into an exam, I fail to see how buying HD notes would be better for you than the textbooks/lecture materials. You also assume that nothing has changed between when they wrote those HD notes and when you study the subject.
Probably not worth it :/
Unless you're really low on time and just need a good set of notes.
Don't different unis have slightly different course structures anyway?