Earlier quoted context omitted.
> it would be trivial to trace anybody putting money into the mixing service Not really; you can't tell which bitcoin addresses belong to a mixing service and which ones don't; it's all just transactions from one address to another. You may know the owner of the originating address, but not the receiver. There are also mixing service that run as Tor hidden services just like the illegal marketplaces themselves, so yo…
They could try using a mixing service themselves. Once they do, they have know that where they sent money is part of the mixing service and can start from their. Not to mention the fact that mixing services likely have statistical properties different from other transactions.
The mixing service can create as many wallets as it wants, each wallet could be used for one user, for one transfer if necessary too.