You were almost on the right track with the withdrawal:)

You can sell your SBD or STEEM coins for Bitcoin through blocktrades. Just click the down arrow next to the amount you have in your wallet and choose Sell.

It will redirect you to blocktrades. Input the amount you want to sell and the receiver Bitcoin address. Than click on Get deposit address. Read the info what shows up.
Copy the memo code.

Go back to your wallet and make a transfer (similarly, click the down arrow and choose transfer). The recipient is @blocktrades also paste the provided memo code to the memo section!

Steemit will ask for you ACTIVE private key to validate the transaction.

Than wait until the STEEM, than Bitcoin blockchain clears your transaction. From that point, you can sell your Bitcoins to local currency.

Done!:)

Finally, the email address for your inquiry is :
support@blocktrades.us

You can give them this as a transaction ID: e0f8ea5fd1cb93d816818e3eeae4841bed29dce3