is photographing graffiti stealing?

I'm going to equivocate on this question (because it suits me to). So yes it is and no it isnt. What the?!? 'Legal' graffiti with registered artists that can be readily found is off limits to me, they have played by the rules and created a property right to be respected, I wont touch it. Anonymous taggers, stecilers, etc are a whole lot harder to figure out.

This is my rule. If I've got a tag I can identify (and that's something I'm not good at) I'll try and attribute it and in the event I made some cash and found the creator I'll gladly come to some arrangement.

But then again how do I come to an agreement with an anonymous person, who do I make the cheque out to or whose bank account do I deposit the funds into. How do I know the person purporting to own the copyright to the image in fact does. If all I have is a false identity how do I get recourse later if another person claims/proves ownership? I see some really difficult practical questions arising here.