Gallery, Forum, and the Market Place are all separate areas on this site.  I don't see why an infringement in one area results in a ban in all areas.

If someone is repeatedly posting images in the gallery that violate the TOS but they are in good standing in the MP and the forums, then why not just remove their ability to upload images to the gallery?

As for giving a warning or banning someone who is in good standing because they have posted an image that in their view was "of age", but to a moderator it wasn't, is dumb.  Remove the image and be done with it. Or implement a temporary "moderated upload"  period for the person so that all their gallery uploads have to be pre-approved before being  allowed public viewing.

Then once they've proven themselves, allow them to upload directly to the gallery again.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi