Can I use models on CivitAi for commercial purposes?
There's a different license for each model. You can check it below the model description, as shown in this picture:
Yes, so, the "licenses" shown in @Vohr's image are guidelines set by the uploader, but Civitai won't enforce any of these - we just suggest they're followed as a courtesy to the uploader.
I disagree that these are "guidelines". It is irrelevant whether Civitai enforce the licensing settings, especially since enforcement would likely require authorization from the creator, as it's not part of the ToS.
The models posted by users are in themselves creative works protected by intellectual property law, even if derived from training on SD (or other derived models). Using models posted here for commercial purpose is possible if the creator licenses it for such purposes. I won't get into the validity of the licensing settings made available by Civitai, but the suggested approach is to reach out to the creator to obtain permission to use their model for commercial purposes.