Jagex recommended the official trading platform in Old School RuneScape
Jagex discussed their recently proposed OSRS "collaboration plan" on Old School RuneScape's blog. The partnership program is a new program that Jagex is trying to introduce, which allows players to spend real-life money through third-party vendors to buy RS Gold in games. Jagex believes that these partnerships with third parties will incentivize new participants to join OSRS.
Especially consider that members who play games for free cannot access this exclusive content unless they get more cash or wait for an undisclosed time. As a free game, they will not directly affect the gameplay. But subscription players will enjoy more rights, and free players will need more time and energy to play in order to get a better experience.
For example, the partnership between Jagex and Twitch. This website provides an official trading site for players who purchase Twitch Prime. They usually choose "Buy OSRS Gold" on https://www.rsgoldbuy.com/.