Is it ok to buy a used crib?

Not recommended. Many older cribs are no longer deemed safe under new safety rules, so it is safer to buy a new crib that meets current safety standards.
No. These days, used cribs are a problem. Drop side cribs are off the market, and considered dangerous. Most old cribs have unknown history, whether they are distressed or not, have loose slats, or weak joints. Any deviation from strict construction rules can be dangerous for the baby. This is one area that a new baby item is better. If you are sure of the history, maybe. Otherwise, probably not.