Nothing to worry about. If you see it, it’s only in testing phase. Nothing’s been done with the Zip/Postal Code. Can’t trace you or anything. You can’t receive FFXI Gil Ads with just Zip/Postal Codes. They also have no plans to give your Zip/Postal or any of that shit to RMT, according to the AH staff.
When you enter a Zip or Postal Code, it gets converted to a city or town and it’s the town/city that gets stored.
Plans.. I don’t know.. maybe tell you in the future who plays near u? Perhaps really send you RMT ads? No, poster near where you live to buy GIL! haha!
So yea, they do look after your privacy and didn’t plan to sell the infos. Unlike Facebook  (Well, I think they reverted back to the old ToS.
Where would you work?
Which workplace would you rather work at to produce good games.