I would highly recommend rsorder The company has been in operation for a few years currently and I've been with them for quite a while. Although payments may be delayed, they're always offering support for inquiries and follow-ups. I believe that OSRS is among their biggest communities. And they've even launched marketing tools for games. I'm sorry that I'm not able to provide the URL right now However, it should be under Runescape.
My suggestion is to exercise care when buying and selling online, as there are always risks associated with online transactions. Do not make any direct deals, and always involve an experienced middleman.