There's actually about three ways to sell/convert your steem dollars. You can convert them to steem internally (which takes one week). You can sell them for steem on the internal exchange at steemit.com/market (instantly). And you can transfer them to an external exchange like Poloniex or Bittrex and sell them for bitcoin (instantly).
RE: A reason to hold Steem Dollars - 10% RISK FREE INTEREST PER YEAR [Newbie Edition]