Isn't the problem with an eBay style market that Jagex will contact them and tell them to stop? The reason you're not allowed to sell Runescape stuff on eBay is because Jagex contacted them and told them not to let people use eBay to violate the Runescape ToS. With a forum style marketplace, Sythe is in the clear, since they never really participate in the sale, they just host it.
Unfortunately, if this actually does come into reality, I can see it becoming as popular as Sythe's market, due to the fact that it will discourage scammers.