When it comes to bitcoin trading sites and services you really need to take a review on the site you want to trade on. Why? Because there are way to many sites, that will either scam you, or close down after a while. For example Mt.Gox, this was a big bitcoin trading site, and just decided to close one day, and they also got problems because of some manipulation on the bitcoins market price. This is why you should always take some reviews, before you send money or bitcoins to a trading website.
I have tested a few sites, also some Altcoin trading sites, where you can buy and sell other crypto-currencys as well. Most of them don’t even exist anymore, and i have also lost a few dollars and bitcoins.
The only one i have always trusted is Bitstamp. On Bitstamp you can buy and sell bitcoins in realtime. This service is running since a long time, they serving the bitcoin industry since 2011 and they are the first fully licensed bitcoin exchange. Its probably the best bitcoin trading site so far. They also have a great support, that will always help you.