Short answer: Notes are worth buying only if they are readable, accurate and match your syllabus. Read sample pages first.
Readability
If you cannot read the sample pages comfortably, you will not revise from them.
Accuracy and updates
Check the year and whether the notes cover the current syllabus.
Make your own too
Notes you write yourself help memory more than any bought set. Use bought notes to speed up revision, not replace understanding.
Frequently asked questions
Can I check sample pages first?
Look at the photos and ask the seller for more pictures through chat.
Are notes a replacement for books?
No. They work best as revision aids alongside a main book.