I have bought and sold hundreds of used batteries, and have had the good luck of not getting any defective ones. I usually buy them as part of a package, but the owners always tell me if they are not holding a charge, have been damaged, or if they are puffed. If there are any marks on them, or charge unevenly, I usually wont buy them. If you use Paypal to buy them and they turn out to be bad, you can always file a claim if the seller refuses to take responsibility.
You can reduce your risk by buying used batteries with a low cycle count that are still under the factory 6 month warranty. You can check by serial number. Just make sure they werent damaged in any way.
Used OEM DJI batteries are better than any new or used generic battery. I never buy generic batteries or props.