I'm more of an individual buyer than a seller, however my primary source for RS Gold is RSorder. They're basically a trader-to-trader site, which means that you're not purchasing or selling with the corporation instead, you're dealing using independent traders on the RSorder's system.
Here's what I believe are the benefits in doing business directly through a business:
Increased competition between sellers push prices down
They can't be charged whatever they like.
More of a community
A wider range of personalized deals
I'd suggest they're one of at least the most secure sites available. There is an escrow mechanism for trades which means that when you purchase an item, you have to send the money for the purchase to RSorder. They notify the seller who contact you to schedule the delivery. After the gold is delivered, you must confirm that you've received it from RSorder and they'll transfer the cash for the buyer (if you don't verify, they'll release it within a couple of days).
Before you actually trade with the seller, you can find out the feedback left for them by past buyers. This is quite helpful. It's simple to discover which sellers are the most trustworthy and reliable are, specifically for buy cheapest osrs gold.