Three questions - UI and Markets

[Niwashi]

Welcome to the game. Glad you're enjoying it. Oh, and it looks like we're on the same server. Maybe I'll see you around.

1. I noticed my hotbar settings wasn't updating when I switched between the two. I googled around, and found out people have been wanting this since 2013, is there any news on when this will ever come out?

No. There was a rather vague acknowledgement of the problem a long time ago, something along the lines of "we're aware it's an issue" but no clear answer as to whether they're going to do anything about it, or when.

2. I noticed people were buying Allagan Bronze Pieces and other things like it from what I can assume are vendor trash, for more gil than you can sell them for. Why?

People who are going to transfer servers will sometimes buy those. There's a limit on how much gil you can take with you while transfering, but you get to take all your inventory items. So if a character is too rich to keep all their gil during a server transfer, they'll buy up things like those Allagan coins as a way to convert their gil into something they can take with them. They're often willing to pay a bit of a premium for the chance to do that.

But for normal purposes, if you're not transfering servers, you're right. Those Allagan coins are vendor trash, just a way to make extra gil be an option when giving a choice of different rewards for a quest.

3. I think I need a retainer if I want to sell things on the market, at what level do those unlock?

Yes, you'll need a retainer. Retainer access is unlocked by the level 17 Main Scenario Quest The Scions of the Seventh Dawn.

Edit:
Oh, and this wiki page has a good listing of when and how lots of other stuff gets unlocked.