A gold seller is someone who sells in-game gold for real currency (€, $, etc.). Sometimes extended to cover someone who sells accounts or virtual assets (items). Gold selling and gold sellers are also sometimes known by the acronym RMT for Real Money Trading/traders.
As gold sellers usually announce their prices, websites and similar services (such as power leveling) in chat channels, they are also often referred to as "gold spammers". The chat channel most used for this is the trade chat. As many players consider these "commercials" to be extremely vexing, there is made a number of addons to block them. However, recently, many gold sellers prefer to offer their gold through established marketplaces such as Eldorado.gg for such practices instead of resorting to spam.