I'm assuming that by selling you're referring to RuneScape Gold for sale. Contrary to what is commonly believed most RS Gold Sites are not "farming" their own gold. They do not have a group of low-paid workers who just click on the Coal rock in order to earn cash. Instead most Gold Sites buy the gold from other players, and then sell it for a profit.
The RSorder website is basically an "GOAT" RS Gold Seller. He's been operating an online gold store since February 2007, which is 12 years ago. I can tell you for certain that he purchases gold from major stakers as well as various other companies, and sells the gold on his website. This is how the business operates, and it does not make sense to recruit people from countries in the third world to run a "goldfarm" producing between 100 and 200k per hour.
Be on the lookout for Chinese websites like the ones that have been featured in this thread. They usually employ more illicit methods to earn gold, including botting. They also often advertisements in-game with bots that draw a lot of attention Jagex as well as ruins the game for everyone. If you plan to buy runescape 3 gold, don't help the websites that advertise in game.
Best of luck!