The most reliable place to purchase can be found at RSorder.
RSorder provide the most secure trading and the most lucrative deals for buyers searching for old school runescape gold of all the sites that I've tried. There are a few reasons why I'd recommend them to anyone searching for the most reliable place to purchase runescape gold
Highly skilled (they've been in company since)
More than 2 million registered users (just proves that it's a reputable site and that getting deals and discounts is effortless since there are a lot of sellers to pick from)
Safety (PlayerGuardian escrow system, Sift Fraud Prevention, Seller Rankings etc)
Market Price Tracker, which lets buyers check the price average of from the old school runescape that is being sold on the website (means that you are aware of the amount you're supposed to pay)