if the image was an urban renewal project and they hired an artist to do it, then it's protected by copyright and you can't or shouldn't try selling it. especially as a stand alone, often those are signed some place.
if it's scrawl on a building, its not likely that the vandal will come forward, so your probably safe there. i think if its a part of a scene, i think its ok, but don't quote me on it.
---Mike Savad
MikeSavad.com