I also use coinmarketcap to know the coin price, it is just a benchmark and can not be used as a reference. The actual price is the price that is on Exchange.
Coinmarketcap just the easiest way to look after the altcoin price. But just looking and not to use it for trading reference.
The exchanger price is the real price if you trading. Wherever you trade and always use its price to buy or sell.