Short answer: Buy new when the syllabus or edition has changed, when you will write in the book, or when condition matters more than the saving.
Syllabus or edition changed
If the latest edition differs from the older one, new is safer.
Workbooks and tests
Anything you will fill in or attempt under exam conditions is better as a clean copy.
Gifts and keepsakes
For gifts, condition and presentation matter, so new is usually best.
Otherwise consider used
For stable books of the right edition, second hand can be good value.
Browse new or second hand.
Frequently asked questions
Is second hand ever better?
Yes when the edition is unchanged and the condition is good, since it costs less.
Where do I find both?
Browse new and second hand books on Bookaao.