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No you don´t need Poloniex if you use blocktrades, only if you want to store your BTC´s there, or you want to buy a cryptocurrency, which is not supported at the blocktrades exchange. You can even use a deposit adress at the blocktrades exchange of a site to sell your BTC´s for Euro/USD, such as anycoindirect.eu. I will write another tutorial how to do this.

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