Upthread there's a link to an Ars Technica article [1], which has this line:
> He even ran a program called a "tumbler" to route incoming Bitcoin payments through a complicated series of dummy transactions, so as to make them infeasible to trace through the public Bitcoin blockchain.
I think what he's saying that because all money went into one account and out of one account, from the outside you couldn't tell who was sending money to who. It means I can pose as both the buyer and the seller, let silk road take a cut, and turn a big block of known bitcoins into small chunks of unknown bitcoins
*Hypothetically asking