there is no way to change your username.

And what you are suggesting is RMT, which is extremely against the TOS of this game.

I would suggest staying on one class until you hit at least 50 (or 60) then branch out so as to not spread out your gil purchases in game on too much equipment. Level crafters and gatherers and work on your retainers so they can bring back things for you to sell. Do treasure maps once you can get access to them and sell the rewards from those.