Do exchanges charge commission for a buy and a sell?

[btctraderr]

So when they say trading fees are like 0.3% they're actually making 0.6% a trade?

[marcovaldo]

No, it is only 0.3 %.

For example, if you buy 100 usd of btc, you will pay 0.30 usd as a fee, and then get 99.70 usd converted into btc at the current price

[DannyHamilton]

Exchange has 0.3% exchange fee

user_A has $100
user_B has 0.1 BTC

user_A places BUY order spending $100
user_B places SELL order selling 0.1 BTC

current exchange rate is $100/BTC

exchange receives $0.30 (from user_A) and 0.0003 BTC (from user_B) in fees.
exchange transfers the BTC and USD between the users.

user_A pays $0.30 fee and receives 0.0997 BTC
user_B pays 0.0003 BTC fee and receives $99.97

So, depending on how you look at it, yes you could say that the exchange essentially receives 0.6% in total fees from the trade.

Another way to look at it is that the exchange handled a total of $100 AND 0.1 BTC in value.  They took 0.3% in fees from the total value that they had to manage.