this question has been nagging at me for ages. one purchases BTC from, say, localbitcoins, then send to one's market btc address. No-one knows who owns that target address - or even that it is associated with a DNM. In the event the site gets siezed THEN the cops could see addresses and associate them with a useraccount and the origin address - but only if the site is compromised. If one changes one's market address after every transaction then there is no record of who controlled that target address. So why bother tumbling?
Allow me to re-post a small rant on why "not tumbling" is not a big deal:
"There is so much paranoia surrounding tumbling. I'll preface this by saying that I do tumble my coins, but is it necessary? For buyers, not really. As a buyer, if you don't tumble your coins, nothing will happen. Blockchain analysis has been talked up on dark market sites as a nearly omniscient technique that can identify anyone easily. Reality: using blockchain analysis to identify unknown people is simply impossible. Far more important is encrypting your address.
Picture yourself as LE trying to identify an unknown suspect using nothing but the flow of coins through random bitcoin wallet addresses. You can't tell if any given address is a private wallet, belongs to a dark market, to Coinbase, to LBC, or anything else. You can't subpoena the dark market. You can't subpoena all known bitcoin exchanges on the mere hunch that a wallet might belong to one of them. Even if a DNM user buys coins in their own name through Coinbase, sends them directly to a DNM, buys something, then sends any remaining coins directly back to Coinbase to cash them out, the coins still aren't fully traceable because all DNMs have internal tumblers. The entire concept is absurdly far-fetched. If blockchain analysis were truly as powerful as some like to think, tons of vendors would be dropping like flies and rounded up within weeks. Reality: likely fewer than 1% of darknet vendors have ever been caught.
You will never see blockchain analysis mentioned in a vendor indictment as hard proof that DNM Vendor X was cashing out coins using Service Y. Not going to happen. One of the Ulbricht indictment documents explicitly mentioned that the SR1 internal tumbler rendered blockchain analysis pointless.
Why bother tumbling coins, then? For vendors anyway, blockchain analysis could theoretically be used after-the-fact as circumstantial evidence. First identify a suspect behind a vendor/buyer account, then subpoena CoinBase and see if they have an account there, and what wallets are tied to it. But remember, all vendors to date -- without exception -- have been caught through normal police work, package profiling, and vendor slip-ups. Not blockchain analysis.
The way some folks here talk up tumbling promotes an extreme sense of paranoia among newbies who don't know any better. You didn't tumble your coins? Guess what, nothing is going to happen."
Rant off.