I'm more of an individual buyer than a seller, but my preferred source for RuneScape Gold trading is the RSorder. It's essentially a trading site, which means that you're neither purchasing or selling through the company, but through independent traders using their system.
Here's what I believe are the benefits when doing business directly through a business:
Increased competition between sellers push prices down
They are not able to make any kind of charge they wish.
More of a community
A wider range of personalized services
I'd suggest they're one of at least the most secure sites available. There is an escrow mechanism for trades and when you buy an item, you have to send the money for the transaction to RSorder. They then notify the seller, who contact you to schedule 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 the funds within a couple of days).
Before you actually trade with the seller, you can also review the feedback provided to them by past buyers. This is quite helpful. It's simple to discover who the most trusted sellers are, specifically for RS 2007 Gold.