Engineering is one of the most powerful professions in classic with access to both trinkets and consumable items. Most of the items are only usable by engineers, and a lot of the gear is also BoP, but the consumables can be traded and are extremely useful.
Why would engineers buy something they can craft?
To save time. Rogues are very willing to buy Thistle tea, even if the recipe is incredibly simple to get. I expect the same happens with engineers as crafting requires a lot of time. Just buying from the AH is significantly easier.
Consumables and PvP
Engineers can craft a ton of consumables. This time around we will only focus on a small subset of them for PvP reasons. Grenades are the man category we care about as they provide damage and CC that can be aimed and is extremely useful in small and large scale PvP.
Iron Grenades
Iron grenades are used in PvP and are great in small scale PvP. The recipe is trained from your trainer in you r major city or Tanaris and requires skill level 175 to craft and 160 to use.
Thorium Grenade
Thorium Grenade is the bread and butter small scale PvP grenade. It is extremely useful when fighting 1v1 or in small groups. Demand may be a little lower with the decline of wpvp, but it is still extremely strong. The grenade requires 260 in engineering to use. The recipe is sold by the engineering trainers in your factions major city.
Goblin Sapper Charges
Goblin Sapper Charges is the number one grenade for large scale PvP. You need to be a goblin engineer to craft it, but any type of engineer can use it, as long as you have a skill level of at least 205. You get the recipe from goblin engineering trainer.
Discombobulator ray
The last item on the list is the Discombobulator ray. This is a 5 charge item that has the primary use case that it dismounts someone when used on them, making it very useful in PvP. The schematic can either be bought on the AH or you can get it from Gnomeregan by getting the delta access card down to Mekgineer Thermapluggs room.
Crafting and auctioning
All of these items are volume items to some degree. The Thorium grenade is the one with the widest use case. As always I suggest experimenting with different crafting amounts to figure out what works. For posting we will use a wide variety of nice even stack sizes to post them, and we will use the crafting cost as the basic price source.