I have no intention of destroying the local city markets. If you check my diagram on the right side under Market Stands, the third point states "Allows trading on the World Market in addition to local city". My goal was to give sellers incentive to renting stalls around the world based on what they were selling. I figured if they were dealing with high volume or valuable merchandise the proposed tax system would spread them out. Anything they are selling on the World Market would still be available on the local city market as well.
Well do I understand your intention. ^_^ I know a lot the ideas I've had, well ... end up with dire consequences or exploits that I hadn't originally intended. So, I'll poke around at your idea and see if we can't find weaknesses and ways to patch them up. I think that is a good concept, that if well-implemented could obviate the need for a global high-end auction house as I've proposed.

I can get on-board with the thought that items available in the world market stalls are also available via visiting the local market wards.

The question I still ponder is whether the world market stalls can sufficiently supply all of the needs of a server. If so, there is no need for further supply, thus rendering the regular retainers standing around the market wards with nothing to do (I suppose they could comprise the "dollar stores").

Working from 600 market stalls (sounds about right to me), and then assuming each retainer (because of advanced level) can sell 15 types of items, yields 9000 item-selling-slots.

A quick perusal through YG gives the following for items:
weapons : 220
armor : 1553
tools : 121
food : 279
potions : 32
crafting : 1534

So, currently in game, there are around 3739 different items. This means that on the world market, 2 or 3 of each item type could be listed. Going further, supposing that many of these items are spectacularly non-valuable (river sand, anyone?), it is conceivable to list many item types 6 to 8 times or more.

In any event, most any item a player could want could be made available via a world market stall. If pretty much all items are available via this method, and all can be searched and purchased from via a single NPC, there would be no viable reason to place items on retainers standing in a market ward.

I could see this scenario as spelling the demise of the current market area system.

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So, what could correct this, and allow world markets without wrecking our current quaint system? Here are some incoherent (and likely ill-thought out) ideas.

a) Delay world market until the mail system is implemented. The seller only sends purchases via Ultra-Super-Saver shipping (i.e. Sick Chocobo), which requires a 1 hour delivery time.

b) Tax rate of 25% for items on the world market. Sure, you may have global presence, but can you stomach the costs of all that advertising?

c) Items from such a long distance away are occasionally stolen by bandits during delivery. Nothing beats the assurance of receiving goods directly from the hand of the retainer.

d) The shipping department occasionally mixes up the order and sends the wrong item. Within the box is a black pudding. It attacks.

e) World Markets are only available during certain times (maybe one hour per Eorzean day). All other times show advertisements for the amazing ginsoo knife (watch it cut through this tin can!).