I'm more of an individual buyer than a seller, but my preferred source when it comes to RS Gold trading is RSorder. It's essentially a trading site, meaning that you're neither trading or buying with the company, but using independent traders on the their system.
Here are the advantages I see in dealing directly with the business:
Increased competition between sellers push lower prices
They are not able to be charged whatever they like.
A wider range of personalized services
I'd suggest they're one of most, if not the most secure sites available. There is an escrow mechanism for trades which means that when you purchase the item, you pay for the transaction to RSorder. They then notify the seller, who contact you to schedule the delivery. When the gold has been received, you'll must confirm that you've received it from rsorder and they'll then release the cash for the buyer (if you do not confirm, they'll release it within a couple of days).
Before trading with the seller, you can find out the feedback left for them by past buyers, which can be quite helpful. It's simple to discover which sellers are the most trustworthy and reliable are, particularly for gold.