If you are selling content for commercial use and that content features an actual human subject, you must obtain a model release from all individuals featured in the content. This includes (but is not limited to) individuals that are photo-realistically depicted, such as in a model scan or texture and individuals used for modeling reference.
A model release is written permission from an individual that grants an artist the right to feature his/her likeness in any work intended for commercial sale/use. Any models that you use must specifically grant those rights. Additionally, there are specific release requirements for child models and nude models:
Children (under the age of 18) must have a parent or guardian sign the release
For any content that depicts nudity, you must not only obtain a signed release but also a picture ID to act as proof that the model is at least 18 years of age.
RELEASE REQUIREMENTS
Artist information
Your full name as listed in your TurboSquid payment information.
Signature
Date
Details
Information about the end creation and what potential end-uses exist.
If you are creating content with nudity, your release must explicitly state how the content will be sold and what potential end-uses exist–no exceptions.
Model Information
Full name
Date of birth
Full address
Email address
Signature of model or parent/guardian if under the age of 18
Date
Witness Information (required for minors and nude subjects)
Full name
Signature
Date
The witness must be over the age of 18 and cannot be the artist, the model, or the model’s parent or guardian.