Example: Halion drops Waist pieces. What does this mean? You can bet your ass that Belt Buckles will sell well immediately following the release. My server sees sales of
A New raid, with new boss drops USUALLY also means increased demand and raise in prices for: Enchants, Gems, Leg Armors, and other Item Enhancements, BUT look deeper into it. Halion drops NO Weapons and No Leg Slot Armor, so thinking you will get a spike in leg armor sales because of the new raid is just wrong. No Weapon drops means no big rush on high end weapon enchants. You should be focusing extra resources into enchants and item consumables that will go on the new drops, like belt buckles and enchants for what items are dropped.
Mainly Tank, DPS, and Heal chants for Feet.
Spellpower and Agi based DPS Chants for wrists.
Leather Caster Hand Chants.
Then throw in factors like increased population online after a new patch and sales in others areas can explode as well, simply from more people being on this week looking to run the new raid. Now is the time to make a lot of profit while you can and unload some things that have been dropping in price, like Gems. Gem Prices have skyrocketted back up with the player population spike as well as the Raid items dropping with empty sockets.
All in all, read you patch notes, plan ahead, use your brain, and then enjoy the sound of that gold hitting your packs.