You are correct. The price you sell at will always be lower than the market price and conversely the price you buy at will always be higher than the market price - that's one of the ways the exchanges make their money.
Well, take the popular exchange, Coinbase, for example: You won't find many day traders using this platform, why? ... their fees are too high! Coinbase has a really friendly user interface and is very simple to use and navigate for first time buyers, but each time you sell/buy you get hit with a 1.5% fee if using a bank transfer, and a whopping 4% fee when using a debit card.
So how do the traders get around paying these fees? They'll use an exchange such as GDAX (I have no affiliation, I swear! ). GDAX allows you to buy/sell using Market price with a fee of just 0.30% - this means you agree to pay whatever the current buy/sell price is instantly. However, the traders avoid paying ANY fees at all by using a Limit price to buy/sell.
....let's say you wanted to buy and the current market price is $400 - you put a BUY Limit price under this, say $395 when the price comes down and hits $395 your purchase is automatically triggered without any fee. If of course the price doesn't come down then it won't get triggered. With the volatility of cryptocurrencies the traders know that in most cases their Limit orders will get triggered and with 0% fees!.
Similarly, if you wanted to sell without any fees you'd put a SELL Limit order above the current market price.
However, all the exchanges have a spread between the buy/sell and current market price ... it's how they make their money.
Of course, using a Limit order means you are placing it in the opposite way you want the market to go, however, the traders will gamble on the price correcting itself eventually (sites like GDAX have great charts to help you get a better feel of trends).
GDAX is actually owned by Coinbase, so once you have signed up to a Coinbase account you can then open a GDAX account using your Coinbase credentials and transfer your coins/money wallet between the two, hassle free.