I don't see how someone can get traced until his real identity gets found because of buying coins in poloniex. Let's say our guys buy BTC in localbitcoin or whatever, uses helix to mix the coins, then opens Tor Browser, then bitcoin core, this should make the transaction to poloniex anonymous, to access poloniex VPN is always used (im not sure if Tor works in poloniex), then you get your XMR, and theorically that's all since XMR doesn't need a mixer right?
So how can they trace our guy?
Actually, there are two different points.
1) Why should one AVOID giving one's identity to Polo when buying monero ? This question only makes sense if you think somehow that the simple fact of buying monero makes you a suspect. If that is the case, then your liberties are in an extremely bad shape. If the very fact of using a crypto currency makes you a suspect criminal, then the only thing you can hope for, is that half the world uses that crypto, so that half of the world becomes a suspect. This is a matter of civil responsibility, to use all things that may "make them think you might be a criminal". Set up a tor relay, use monero, use GPG, .... to fight for your freedom.
2) With monero (contrary to bitcoin) there is no reason to hide your identity when buying it, if you don't want people to link this act of buying monero with whatever you're going to do with it afterwards. That's exactly what the crypto of monero is made for !
Of course the crypto can fail, like the crypto of bitcoin can fail. Dark market activities are of course always a risky affair, and the biggest risk will not come from using monero, but from actually "getting into contact" and "obtaining stuff" there. The whole idea of monero is not so much to stimulate dark markets, but to allow people to get their monetary privacy back which they lost partially with banks, and totally with bitcoin. That privacy doesn't need to be used for illegal stuff. You can just as well appreciate the privacy when doing things that are allowed by the laws that states have decided to impose upon to you. But it is true that dark markets are a particularly well suited test bed for the solidity of the privacy. So there's a symbiosis between all privacy-enabling technologies on one hand, and dark markets on the other. The privacy-enabling technologies find in dark markets a great test bed, and dark markets can appreciate the privacy technology to allow them to escape the prying eyes of law enforcement. It is a win-win situation.
