The Private Store feature in Aion is equivalent to the Bazaar system in FFXI and no doubt a few other player-merchant systems in other MMORPGs. Players select items in their inventories, associate prices with those items, and click Start. Now they are their own traveling salesman.
Except they can't move. In Aion, once they open their store, they pull out a magical stool from thin air and plop their tuchuses down. Roaming merchant is not a valid playstyle. "Well that's ok," you might say, "Players can set up shop while they go AFK for a few hours."
Except they can't AFK for longer than 30 minutes. Originally, players who were AFK for a set amount of time (I'm going to guess 30 minutes, but it could have been 60), were disconnected from the server. Nothing new here. Players who had set up a Private Store were immune from this AFK timer. This makes sense--how else would player merchants make any money?
With all the AFK merchants on the server, queue times began to rise, and clever players realized they could set up a mock Private Store to bypass the AFK timer should they need to AFK for a bit. Now NCSoft had legitimate player merchants and players who were taking advantage of the Store mechanic. They decided to extend the AFK timer to Private Stores. "Let them put their goods on the AH then, and spend their time adventuring," you exclaim.
Except characters only have 10 slots on the "Broker", AKA Auction House. So one of the reasons to limit AH slots is to get players to use the Private Stores. But who wants to actively sit at their computer, watching all the Kinah spam bots? Not I.
I can't really say what the causality is: there is some limitation to having unlimited slots on the AH so players were given the Private Store; or the Private Store was an original feature and AH slots were limited in order to facilitate its use. However, I do know that the Store is in the game in order to let players participate more in the economy. That is one of its functions.
Its other function is to provide buyers access to goods in otherwise remote locations (assuming someone has set up a PS in that location). I can't tell you how much Gil I made setting up next to BCNM fights. Nor can I tell you how glad I am to find players selling what I needed outside BCNM fights. Would those merchants sit at those instance portals, staring at the wall all night long? I wouldn't.
Going back a bit to the "unable to move" attribute of the Private Store, another function emerged from the Bazaars in FFXI. I'm assuming it was not intended, and I'm assuming it wouldn't have surfaced if players were always stationary. The Bazaar acted as a trophy rack. High level crafters would show off their High-Quality synths (Aion also has a HQ-centric craft economy; I'm sure I'll write about that once everyone starts bitching about it). Players could show off rare gear they couldn't equip on their current job, or rare items they found that didn't have a market. Showing off is an important aspect of any persistent social game.
Another quirky detail is that price per item is not displayed to the buyer. E.g. if a player wanted to sell 14 Pigs Feet, at 675 Kinah per foot, then the customer sees a stack of 14 Pigs Feet for 9450 Kinah. If a customer wanted to know the PPU, he must shift-right click to break the stack, type in 1, and look at the price of the single foot in the Shopping Cart. That is a HUGE barrier to entry for a buyer. To combat this piss-poor UI, I've been putting up singles right next to stacks. But I run the risk of someone buying my "For Display Only" item.
You also have to right-click on players, select View Store, and run up to them to view their wares. It's very time consuming to survey all merchants.
The only "good" features of the system are: item layout is preserved (as far as I can tell), and a custom message appears above your head when you are selling. I guess I could put the PPU in the message... maybe advertise at 100 Kinah less and chalk up the difference to hidden fees...