The steemit witnesses uses steem prices all over the exhanges and create an average price from

all the prices,the average price will the most common price on all the exchanges at that

particular moment,some witness also use the official steem price showing on

Coinmarketcap.com they use the steem price showing on the website to determine the price which they will set on the price feed on the steem

platform,that is why we need to keep voting in good witnesses that will not make any attempt to

manipulate the steem price for selfish reasons or to perpetrate fraud...