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You can just use mixers to hide your bigger stash or your main account, but it will not be impossible with the correct legal documents, to trace it back to it's source. ^hmmmmm^
this last part is only true if the mixing service that you are using is keeping logs so the government can trace you through the legal channels. but mixers like bitmixer are not keeping any longs at all, and all the records will be removed immediately.
What logs are kept?
Absolutely no logs or personally identifying information are kept regarding your use of the Bitmixer service. All logs are permanently removed from both our website database and from our Bitcoin wallet after your coins have been transferred out of our reserve. All forwarding entries are removed after 24 hours.
although like any other service it is up to you to decide how much they are keeping their word and destroying the logs.