Some of the information from those original posts by Aruspex has changed. Of note, players no longer get 4 and 6 point assertions from the newbie quests. Those are now 3 and 4 point assertions, plus one 5 point assertion (race card).
Also, the Diplomat's Circlet is now the Diplomat's Mask. Same concept though.
I think at launch there will only be 2 cities with Writs. In beta, there were Writs from BC to Leth Nurae, so possibly they will have all the Writs turned on going back and forth between those cities.
Aruspex did another write-up on Regional Trade. If anyone can hunt that down, that should be added to this thread. I'll list what I remember of it, but Aruspex did a much better job of explaining it than I will I'm sure.
Regional Trade involves doing civic parleys with merchants to spawn vendors. These vendors, depending on which lever was used, will sell different things. In Regional Trade there are 3 markets. The White Market involves common items. Players can spawn the white market trader in BC by parleying with merchants, then buy Widow's Lace from the trader. They can then travel to another city, spawn the white market trader there, sell him the Widow's Lace and buy up another commodity to sell somewhere else. The only limit on White Market trading is how much you can carry, as white market goods tend to be heavy.
The Gray market as I recall will only be available in a few cities. These cities will be connected (metaphorically), creating a "silk road" and "spice road". Gray market traders are spawned in the same way as white market traders I believe, but by raising a different lever. The amount of items the gray market trader has is limited and can only be sold in one other city. Gray market items are not illegal, but are subject to taxation if you get caught with them. I have no idea how that plays out though.
The Black Market involves the buying and selling of illegal goods. IIRC, these traders are always spawned, but getting caught with illegal goods carries a potentially heavy penalty. I think Aruspex mentioned imprisonment
Each black market item will require you to sell that item in a certain city. Which cities will buy which items we will have to figure out on our own.
I think that's about it. Hopefully I got it all right
I'll dig around and see if I can find Aruspex's write-up on it. From what I understand, they are working on Regional Trade right now and at the very least, the White Market should be in shortly after release.
Edit: VGTact has most of the write-up on Regional Trade in their FAQ, though it is missing the last 2 sections which detailed the Gray and Black markets.