Similar to the old /tp market, however instead of players renting out the entire shop within the middle circle, the shops should represent categories and players should be able to (for a small fee) be able to place a ChestShop inside. The old shops were too pricey and too few for many players so I suggest this approach.
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The outer ring shops will be completely rentable however, just like the old /tp market and as such, more expensive. Although /ah let's Players obtain quick materials, I think physical shops will be more popular with certain players, especially since items on /ah get pushed off the list after some time. Chest shops also work as advertisement for merchants so that players know who to hit up instead of waiting for a reply in (T
Trade chat.
But the lag! I hear you gasp. Well, you should be able to physically see the Marketplace from spawn, however using that super nifty MassiveGate feature that makes transition from one place to the next seemingly seamless, this shouldn't be a problem since it would technically exist elsewhere.
"Any reason to at least get occasional traffic through spawn would liven things up." -Ethibas T, MassivecraftForums