I'm guessing that by sellers you're referring to OSRS gold sites. Contrary to what is commonly believed that the majority of buy cheap old school runescape gold Sites are not "farming" their own gold. They don't have a horde of unpaid employees who are glued to clicking on the Coal rock to make cash. Instead most Gold Sites buy the gold from other players, and then sell it for a profit.
The RSorder site is one of the "GOAT" RS Gold Seller. The company has been operating a gold website since February 2007, more than 12 years ago. I'm certain that he purchases gold from major stakers as well as others, then then resells it through his website. This is the way the business operates, and it isn't logical to employ people from countries in the third world to run a "goldfarm" producing between 100 and 200k per hour.
Keep an eye out for Chinese websites, such as those that are featured on this thread. They are more likely to employ illicit methods to earn gold, like botting. They also often advertisements in-game with bots which draw lots of scrutiny from Jagex and can also ruin the game for all players. If you plan to purchase OSRS gold or buy osrs ironman account, don't be a supporter of sites that are spamming within the game.