This is true, but if you are trying to arbitrage back and forth to ultimately get more steem, you may pay fees to transfer the steem to a third party exchange then fees for the trade at the exchange then again back to steemit, plus some exchanges have been really crappy about letting you withdraw steem lately when it is up in value - probably to force you to convert to BTC so they get a larger miner fee from the trade, and again when you transfer out. These fees add up fast - and differ of course, so do your own due diligence to compare apples to apples... but you may do better in the long run just using the Steemit platform market.