So by posting in the lowlight directives you're directly calling out the owner of this website. And you're telling him to do something about a fraud happening somewhere else that's not his website?
As mentioned above, you should sell stuff pictured with a paper or sticky note that has your username written on it.
Buying on gunbroker is like buying anywhere else. There's always scammers everywhere, it's no different than buying on eBay, etc. Do your own research before purchasing. Also, the pictures posted here are pretty much public so anyone can literally save them and use them. If you don't want others using your pictures, don't publicly post them.