I'm more of buyer than seller, however my primary source for traditional trading in RS 3 Gold is the RSorder. They're essentially a trader-to-trader site, meaning that you're not trading or buying through the company however, rather, using independent traders on the RSorder's system.
Here's what I believe are the advantages in doing business directly through a business:
Increased competition between sellers push lower prices
They are not able to be charged whatever they like.
More of a community
A wider range of personalized services
I'd suggest they're one of most, if not the most secure websites available. There is an escrow mechanism for trades which means that when you Buy OSRS Gold, you have to send the money for the transaction to RSorder. They notify the seller, who calls you to arrange the delivery. After the gold is delivered, you must confirm that you've received it via 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 also review the feedback provided to the seller by their previous buyers, which can be quite useful. It's simple to discover who the most trusted sellers are, particularly for gold.