Can a back of people photo be used for commercial use?

The question is, could the individual recognize themselves, not necessarily whether other people would recognize them. An article of clothing, a hairstyle, the distance from, the time of the day the person might recall being in the spot or place when the photo was taken, etc. And the individual themselves? Would I go after someone using a photo of the back of my head commercially? No. Would someone else? Maybe. At least that's my take on the issue. 

Photos of people whose faces are not visible do not require a model release, so many of these photos are posted to AdobeStock.
However, you should always get permission from the model for commercial use before posting.
There have been cases of photos being posted without permission, including on other stock sites, and this has caused problems.

If you ask whether photos of people's backs or legs have commercial value, there is the possibility that they could be sold, as there are users who want to use them for a variety of purposes.
Since showing the face gives a strong impression of the model, I sometimes deliberately use photos where the model's face is not visible.

If the persons can identify themselves, you will need model releases.