The problem with this kind of mentality, is that everybody and their brother has already thought of this, and the markets end up getting flooded with said "hot" items, and yeah at first, the money is crazy then supply and demand rears its ugly head and the former spikes hard when everybody thinks "AH-HA!" and dumps goods on the market like nobody's business.
You're right, this market only exists for a very short time. But it's going to be an ENORMOUS market with essentially infinite demand.
For about a week, and then it's gone.
IMO, I'd try to find something that's a bit off the beaten path, something that not everybody else is doing at the time, a steady and reliable thing, perhaps some kind of consumables? Perhaps crafting/gathering tools for the Lv50s for all the newbies that are lured in by Stormblood's release? Then they find themselves in Ishgard and the NQ stuff really doesn't cut it when they want to level up NOW and don't feel like waiting months on the Grand Company Turn-ins/Moogle Quests to level them.
Hmm maybe. Bear in mind a new player is still starting from level 1. If you want to target the new player market you might be better off focusing on the 1-30 range and selling tools and levequest crafting turn-ins. Accessory sets are also useful in the 1-50 range as dungeons and quests tend to be pretty stingy on right-side pieces.