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Let us assume for a moment that the Chinese Trading Post will not be separate from the current "global" one. If you weren't aware, all worlds in Guild Wars 2, even EU and NA ones, share the same trading post. That means I can sell garlic to someone in Germany just as easily as someone on the same server/world as me. There's been a lot of discussion regarding whether the Chinese Trading Post will be part of this global economy or its own entity. There is precedent here for both to exist.
For starters, we already have an EU and NA combined trading post. Countries, especially China, will not want money being transferred to other countries, which is why games like Diablo 3 couldn't have combined auction houses. In this case, the money is paid to ArenaNet for purchasing gems only, not between players. So I can't see why this would be a problem in China. Finally, the Chinese government usually creates separate versions of games because it doesn't want to mix with the values (or lack of values) present in other cultures. For example, World of Warcraft create completely different skins for the undead so that no bones would be visible in the Chinese version. They viewed such displays as extremely graphic and inappropriate. Obviously, these objections extend to all interaction with players from other countries, but guess what, we already have such separation of players with the EU and NA pve/pvp divisions.
In my mind, the only way we won't see a separate trading post is if items are somehow changed between the Chinese and rest of the world versions. If, for example, they remove copper from the game in China, then we won't have compatible Trading Posts.
So let's say I'm right, for the remainder of this post, and discuss how best to prepare for another several million new Guild Wars 2 players.
Preparing for the Chinese Trading Post
Stockpile the following every day:
1. Tier One Materials
2. Skill Points + Dusts
3. Crafting Materials
Tier one materials will almost certainly go up in value. The demand for so many so quickly will overwhelm the current supply. This is a no brainer.
Skill points will be needed for converting your tier one materials into tier two. This will be much cheaper Along with dusts and, since we're being thorough here, one of the tier two version for the Mystic Forge.
Crafting Materials will be a must, and lucky for you there are dozens of items to invest in. I would especially focus on anything that is currently flippable for profit. Most likely this will remain true after the Chinese traders enter the market.
If you invest in gems before the announcement is made for integrating Chinese players into the Trading Post, then most likely you will be in early enough to flip later on. Remember, you need a huge shift in gem price in order to flip for profit because of the obnoxious fees involved. If adding a few million players doesn't do it, then nothing will.
Exotics are also probably worth investing in, but they will suck up your inventory space quickly, so I don't recommend them.
Of all the tips I've recommended, Gems are the riskiest. For those looking to save up small amounts and speculate on this change, try placing buy orders on tier one materials, skill points, dusts, and crafting materials. Get a few hundred each day and begin your stockpile. Don't spend more than 25% of your money on this speculation.