If you are using darknet/ dark web marketplaces you should use an anonymous bitcoin wallet to mix or launder/ clean your bitcoins.
The best way to have anonymous bitcoins and leave no traces on darknet markets is by using a completely anonymous bitcoin wallet with coin mixing capabilitys.
One of the best anonymous bitcoin wallets is OnionWallet, one of the oldest and most secure and trustworthy bitcoin wallets in Tor.
Anonymous bitcoin wallet
Onion Wallet has no fee for mixing, except the bitcoin network fee.
Free bitcoin laundry
There is another similar free bitcoin mixer on the tor network which is EasyCoin http://easycoinsayj7p5l.onion/
Or if you are not using tor you can go to http://easycoinsayj7p5l.onion.to/
EasyCoin is also completely free except for the network fee and most likely the most recommended bitcoin mixer in 2017.
How to use a bitcoin mixer
To mix or launder your bitcoins just create an account with OnionWallet or EasyCoin and send bitcoins to the address shown on the wallet page.
Then ideally wait a few hours so you can not be identified using timestamp attacks and then withdraw your bitcoins to a fresh and clean bitcoin address you never used before.
Beware of non-working bitcoin mixers
There are many other bitcoin mixing services out there, but be careful, not all of them really provide good mixing.
Also make sure to only use the urls provided on this site because there are many fake sites.