The reason behind Jagex seizing the "gold4rs" domain was because it contained "rs" and a court ruled it was confusingly similar to their trademark. I'm not sure any big gold merchants would want to trouble themselves with that. That being said, Gold4rs was by far the largest dealer of gold, and while the merchants on here are sizable, they don't really compare. Their targeting them might have been solely because of this.
I've had insider information and this is for sure not the case. There's hundreds of sites with Rs. Thersgold, rs2hot, gpors, rsgoldfast, runescape4u, 2007runescapegold, rskfc the list goes on and on. Still for sale.