I assume this question apply to stock market too? Price on coins are different on different Exchange? However the price difference should not be much, and the price change constantly. Therefore I should not worry about it?
Coinbase has high fee.
LocalBitcoins has scammer. Price is set by the seller? It means price could be very different than market?
Seems like many people use Poloniex, Bittrex?
You're talking about arbitrage. There's so much big money in BTC today - institutional investors with tends of course millions plus the experience and software to capitalize - the you can be fairly certain that there isn't any arbitrage opportunities for the little investor.