Believe the major and obvious issue with very old speakers is surround break down. Many can be replaced.
On not so old, usually there are some small or not so small cosmetic issues. There are also some brands which are not currently popular which can be scored for 1/10th original price or more but still an incredible bargain. Buying used speakers off the Internet can be tricky to know if you will like them. Buying one of these highly discounted ones may be more difficult to resell. So, hearing some at a dealer is the safest way. Unless you buy a well known and popular pair that are just an older model and/or a bit scratched up.
On not so old, usually there are some small or not so small cosmetic issues. There are also some brands which are not currently popular which can be scored for 1/10th original price or more but still an incredible bargain. Buying used speakers off the Internet can be tricky to know if you will like them. Buying one of these highly discounted ones may be more difficult to resell. So, hearing some at a dealer is the safest way. Unless you buy a well known and popular pair that are just an older model and/or a bit scratched up.