I'm more of buyer than seller, but my preferred source for old school runescape gp is RSorder. It's essentially a trading site, meaning that you're not purchasing or selling through the company instead, you're dealing using independent traders on the their system.
Here's what I believe are the benefits when doing business directly through a business:
The more competition among sellers drives lower prices
They can't be charged whatever they like.
A wider range of personalized deals
I'd say that they're among most, if not the most secure websites available. The trades are facilitated by an escrow system and when you buy an item, you have to send the money for the transaction to RSorder. They notify the seller who contact you to schedule the delivery. After the gold is delivered, you have to confirm that you received it from RSorder and they'll transfer the funds back to the vendor (if you do not confirm, they'll release it within a couple of days).
Before you actually trade with the seller, you can also review the feedback provided to them by past buyers, which can be quite helpful. It's easy to determine which sellers are the most trustworthy and reliable are, specifically for gold or rs3 account for sale.