Websites may take real money and fail to deliver the promised in-game gold.

Jagex maintains that RWT encourages macroing (botting), cheating, account hijacking, and negatively impacts the game's overall economy. ⚡ Risks of Buying RS3 Gold via Third-Party Sites

While a massive gray market exists for purchasing in-game currency with real-world money, the game's developer strongly enforces policies against it.

You purchase a Bond directly from the official RuneScape website using real money. Once in-game, you can sell that Bond on the Grand Exchange (the in-game automated marketplace) to other players for RS3 gold.