Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.
The expansion is set immediately after the events of Legion. Two continents have been added within the Great Sea between the Eastern Kingdoms and Kalimdor: Kul Tiras, one of the major human kingdoms, and Zandalar, the homeland of Azeroth's trolls. While the Alliance and Horde will initially travel to one continent respectively, both continents will be available to both factions at level 120.
When assigning a selling price, do not aim for too low a price that would give you too little profit, but certainly do not assign too high a price. Too much greed is never a good thing, and the AH is filled with items that do not sell due to excessive inflation. A very common tactic in the AH is to sell for lower than what another player is offering, and many players make a lot of money that way. Similarly, losing sales thanks to being undercut is never fun. Aim for as low a price you can that will still make you a good profit. If you can keep producing the same item over and over, sell cheaply yet make a good amount of the item, you will reliably make a lot of money by volume. Moving inventory is the best kind of inventory.