When they released RS Bonds the ex-head of the company stated that 40%+ of the players of runescape at the time were buying gold. A couple of years on, this figure is likely to have increased.
They won't ban buyers often because buyers are their customers, continue to play the game and are likely to engage in membership subscriptions and/or microtransactions.
Sellers however, often do not play the game, unless they're a small time real world trader.
Even websites holding gold on level 3 mules which stand in one place, login on a VPS and never engage with players other than trading often do not get banned for many months until a wave happens.
I could write ten pages on both FACTS and THEORIES behind this, but in short it would make no logical sense for jagex to punish the buyer, when they can focus on making it harder for the gold seller whom's only motive is to profit from the currency of runescape.

