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I am thinking about using Bittrex, which I have been using lately. They shown professionnalism and they have a massive volume.

Hi @brainynetwork, I can suggest you to check out Bitshares network also for crypto trading. Bitshares is based on Graphene same like Steem. It is a fully decentralized exchange, where the others are not.
You can also send your STEEM into there and trade it ... check out bitshares.org