Personally I don't buy my gold nor sell any.
But I keep reading about people saying it isn't safe to buy gold because of the high chance of getting banned.
The only site that I have and would ever recommend to buy gold would have to be Sparter.com
Main reason being that you get to decide whether to buy gold from a website (Which usually means a chinese ip) or buy gold from a player (US or Au ip). To do this all you have to look at is the price and who the seller is. Eg. gold.com or susie
Obviously gold.com may be a few cents cheaper, but are you willing to risk your account over a few more cents.
Also when your buying from a player, they would only put up for sale what they have in stock. Eg Buying 5000 gold usually means that they would have 5000g on their character to pass onto you. So as soon as they get paid they would log on and transfer the gold onto you.
The way blizzard would look at it. Transferring gold from chinese ip to a non asian ip would most probably raise red flags and put both accounts in for further review.
So far I've told two guildies about sparter and both have had successful transactions. One was delivered within the day, the second was delivered within 3 days. But it was stated by the seller that longest it would take to deliver would be 7 days.