I would say, not your responsibility.

If possible though, see if you can find reviews for the case that you have (unless it was something like a custom job).

And example, this one has multiple bad reviews saying it leaks.

https://www.amazon.com/Suptig-Replacement-Waterproof-Protective-Underwater/dp/B0774QGG1G/ref=mp_s_a_1_12_sspa?crid=RRM93L7MU4F1&keywords=gopro+waterproof+case&qid=1676914965&sprefix=gopro+waterporrf+case%2Caps%2C178&sr=8-12-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExMTZZMVNEU1pEQ1lIJmVuY3J5cHRlZElkPUEwNDg1MjQ4TlExUUMzSks1TVhTJmVuY3J5cHRlZEFkSWQ9QTA2MjQyODhCNEtXVzczOU0xWCZ3aWRnZXROYW1lPXNwX3Bob25lX3NlYXJjaF9tdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl#aw-udpv3-customer-reviews_feature_div

You don’t want to be selling a product that is defective.