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I coded it ;) The bot makes transaction on the internal market taking current prices. So when you sell STEEM for SBD, the STEEM ask price is taken and the exchange transaction is performed. The previous transaction could be with lower price but the bot does not have time to make a waiting order and instead just grabs the STEEM that is offered :)

Hi @cardboard
Thanks for the explanation, this type of xpres exchange is very good ..

It must be more widespread.