You have applied a flattening filter to the picture. You see this because the histogram got stripped of the blacks and whites, creating this nice effect.

However, for stock, you need the picture correctly exposed. The buyer can apply this effect if they wish. I agree with @Ricky336 on the lack of sharpness. You should have preferred a shorter exposure time for this picture. 1/100s is not enough for moving objects. The eyes need to be crisp sharp.
Besides technical issues, you will need a property release from the creator of the costume and mask. A Model Release is not needed, as the person is not recognizable.
You will need to edit out any logos like here:

...and you will need model releases from these people and all other faces recognizable in the picture:

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