Just received this email after transferring funds from my Agora wallet to my Coinbase account:
Hello,
We've had to close your Coinbase account as it appears you have conducted a transaction that is prohibited (see https://www.coinbase.com/legal/prohibited).
If you feel we have reached this decision in error, please reply with evidence demonstrating the nature of the goods or services on your most recent transactions.
We appreciate your understanding and cooperation in this matter.
Thank you, Coinbase
The solution to all this isn't necessarily tumbling. Because then Coinbase will potentially accuse you of money laundering; they can easily trace a tumbler's hot wallets just like they can trace a DNM's wallets. Tumbling doesn't hurt, but it may not help either.
A workable solution is to withdraw all DNM funds to two successive wallets you control. Paper wallets, MultiBit/Electrum wallets, whatever. There has to be some intermediary step so Coinbase can't say, "aha, a drug site directly sent these coins to this guy's Coinbase account". With the intermediary wallet(s), it could be, "aha, a drug site sent coins to a user, who then sent them to another user, who then sent them to their own Coinbase account".
I understand Coinbase is trying to comply with laws, but the whole thing is ludicrous and will cost them business in the long run.