I'm guessing that by sellers you are referring to Cheap OSRS Gold Sites. Contrary to popular belief that the majority of RS Gold Sites are not "farming" their own gold. They do not have a group of unpaid employees who are glued to clicking on an Coal rock in order to earn cash. Instead the majority of Gold Sites buy the gold from other players and then resell 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 can tell you for certain that he buys gold from major stakers as well as various other companies, and then resells the gold on his website. This is the way the business operates. It isn't logical to employ individuals from third world countries to work in a "goldfarm" producing between 100 and 200k per hour.
Keep an eye out for Chinese websites like the ones that have been featured in this thread. They are more likely to employ illicit methods to earn rs gold 2007, including botting. They also frequently with bots in game that draw a lot of scrutiny from Jagex as well as ruins the game for everyone. If you plan to purchase RS 07 gold, never be a supporter of sites that are spamming in game.
Best of luck!