1. What is a RMT? I keep seeing that thrown around on these forums.
forgot exact acronym think it is Real Money Trade(r). It's gil sellers, powerleveling messages for real cash

2. Is it ethical to buy [Item A] from here [NPC] and sell it at a higher price than what you bought it for?
Ethic?...depends. I don't agree with scamming people but you can't hold the hand of everyone and it's not real life money. And sometimes people don't want to move to buy something from a vendor far away so it can be viewed as a service.

3. Any specific items that are ALWAYS in demand?
hahaha check the market, want steal business? Materia, food....

4. Does it matter which [city] market you sell in?
No (people that are in the actual city pay less taxes)

5. If you are on Coeurl server and you have been on another server, how would you say our economy compares overall?
not applicable to me

6. Any other general tips? I am only level 40 so I cannot sell super rare end game stuff...yet.
Either farm rare material or gather or craft, but if you choose the latest hope you have time (you can do all 3) First of all get 50 unlock everything