Yes, and for this reason, you will require their permission if they can be identified. The exception is in the case of those appearing accidentally in photos that represent a current event or public incident.
There are many apps for third persons to sign their consent for the sale of their image, for example, Image Release. We advise you to use them. Remember that Agora is not a middleman, and as such, you assume the full responsibility for the sale of your images.
In the case of minors under the age of 14, the consent of a parent/legal representative/tutor is mandatory. For children over the age of 14, it is usually acceptable to have the consent of the minor in question, as long as no other legislation is infringed (such as the Organic Law 1/1982, approved on 5th May for civil protection with the right to honor personal and family intimacy and image).
The photographer Yuri Arcurs has facilitated a standard model contract here: http://arcurs.com/wp-content/uploads/Model-Release-unsigned.pdf