Most important thing when it comes to gold buying is, don't try to get the cheapest price. There is a very high chance you will get scammed or banned.
It is also important to know, whether the gold seller uses a proxy server to face trade/send the gold or not. Most of the crappy gold sellers give a fuck and just use their chinese/vietnamese ips.
There is a pretty big gold selling company i'm using for years for various games, they are always amongst the cheapest. We are not allowed to advertise, so i just say it ends with **oga