Think about this: If the site ONLY sells gold and silver, then it becomes much more difficult to demonize those people. Any effort to do so would be interpreted as an attack on justice.
If you do this you dispense with the benefits of having a diversified market and all the network effects that carries.
If I said I wanted a tor trading site that only dealt in 18th century french coins, if another site existed that already allowed me to do that, would you think I was being silly?
My main objection is that your stated concern is impossible to rectify, if you run from all possible guilt by association you will never stop running.