My 2 cents:
Kamadan is probably the most active trade spot these days, but L.A. is still quite active.
I agree with the OP that GToB would be the best place to trade. Oddly enough, if you sell stuff through Guru auctions, it's the place that most people meet to complete the deal, simply because everyone can access it (although not everyone knows they can).
To take the OP's points one at a time:
1 - The 8-people thing is really minor, but it would make life a little easier.
2 - Also relatively minor - you can trade with your own mules practically anywhere - GH being best choice really.
3 - This is the kicker. Some in this thread have suggested that GToB is a "PvP town", but that's a bit inaccurate. You don't need to be a PvP character to get there and no PvP stuff is actually done there. Plus, it's the way to access the Isle of the Nameless to try out things for PvP
and PvE. And, you could trade stuff besides just Z-keys.
The main point though, is that
it is accessible to ALL players regardless of what campaigns they have.
Edit - the most logical thing would be to use the International districts.
4 - also a minor, but helpful, point.
Edit#2 - about the one disadvantage. There are no material traders there, but the only thing you need those for are to check current prices. You can check prices in some other town or your GH (if equipped) before you go. Not a big disadvantage - not much worse than running all over Kamadan to get from storeage to NPCs.
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I don't think you'd need to be concerned about GToB becoming too crowded because, like any outpost, extra districts are added as needed. There are only X number of players in any one district. If GToB did become the trade town, just like in Kamadan, district 1 would be the main trading district - those who want to do other things (like complete Guru Auction deals) would simply go to higher numbered districts.
However, I doubt that it's ever going to happen. For one thing, it's a sort of chicken&egg thing where you need to have sellers in GToB before buyers will go there, but you need buyers there before sellers will go. Perhaps though, I will spend some time there in future.