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BitShares, https://wallet.bitshares.org
Bittrex, https://bittrex.com
BlockTrades, https://blocktrades.us/
Changelly, https://changelly.com/
HitBTC, https://hitbtc.com
Poloniex, https://poloniex.com
UpBit, https://upbit.com/

I usually use BlockTrades if I am buying or selling STEEM/SBD. Bittrex is usually my second option.