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I use the internal market to sell SBD to STEEM a lot when the exchange rate is good. If I was to sell to fiat I would go to Bittrex to get to BTC then where ever to get Fiat.

sorry, let me rephrase my question, I mean to get the conversion rate API haha. so there is an APi call to get the internal market price? I didnt knew that

You can call the price for Bittrex and Binance and average them. This is what I would recommend. Each exchange has their own API and CoinMarketCap has a global API.