Basically they can buy them for a percentage of what they're worth. The seller will get them, say, as a gift card or from a store, and because it's their only means to get Runescape Gold/whatever they're looking to buy, they sell it at a certain percentage (70,80,90) for the currency that they're seeking out to get.
Ideally, these people would usually use paypal, but for whatever reason, be it they're too young and don't have access or they just got it as a gift card from grandma for Christmas (and would rather have gold in runescape than a product on amazon), they'll exchange it at a percentage for someone's gold, paypal, or other various methods they do have access to.