I'm more of an individual buyer than a seller, but my preferred site when it comes to RS3 trade in OSRS Gold is RSorder. They're essentially a trader-to-trader site, meaning that you're neither purchasing or selling through the company, but using independent traders on the their system.
Here are the advantages I see when doing business directly through a business:
The more competition among sellers drives lower prices
They can't make any kind of charge they wish.
There is more scope for customized services
I'd say that they're among most, if not 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 purchase to RSorder. They then notify the seller, who calls you to arrange the delivery. When the gold has been received, you'll must confirm you've received it from RSorder and they'll then release the cash back to the vendor (if you do not confirm, they'll release the funds within a few days).
Before trading with sellers, you may also review the feedback provided to them by past buyers, which can be quite helpful. It's very simple to Buy RuneScape Gold discover who the most trusted sellers are, particularly for gold.