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The easiest way is to use blocktrades to sell STEEM Dollar to another cryptocurrency like Ethereum and then make an account on Coinbase.com and sell it there for $. Or you can send your SBD to an exchange like poloniex.com and then exchange for Ethereum and sell it on Coinbase.

i made a poloniex account which is only got registered not verified yet in the next process i cant understand what i do?

You have to wait for them to verify you which takes a few days. You can use blocktrades to instantly but Ethereum (ETH) and send it to Coinbase address. You cannot sell on poloniex to USD from what I know.

I personally use Coinbase to transfer my crypto into $ which goes into bank account.

is coinbase for SBD selling ?