Short answer: Stable subjects are usually fine second hand; fast-changing subjects need the latest edition.
Usually safe
Basic sciences, classic mathematics and literature change slowly.
Check carefully
Law, medicine, economics and computing update more often, so check edition.
Always check
Condition, missing pages and the edition number before buying.
Frequently asked questions
How do I know if the book changed?
Compare edition numbers and ask the seller or a teacher.
Can I ask the seller questions?
Yes, through the in-site chat on Bookaao.