Here's a good example: Delicate Arcanite Converter.
Personally I haven't seen anyone selling these for months. These cost about 5g to make and can be sold even for 50-500g. If you want to sell them the fastest, I'd keep the price at around 150-200g each.
Alchemy is essential
The schematic cannot be learned from a trainer, but instead you must buy it from a vendor in Winterspring. If you don't have engineering, you can sell the schematic instead for about the same price as a converter goes.
Starting with materials:
- 1 x Thorium Bar
- 1 x Arcane Crystal
For the Delicate Arcanite Converter you need:
- 1 x Arcanite Bar
- 1 x Ironweb Spider Silk
There's more profit to be made!
As you can see, buying Arcanite Bars from the AH is not always very wise, so you should look for an Alchemist who can create some for you.
Selling these along with the converters can make some nice gold often!
Arcane Crystals too expensive? Worry not!
If Arcane Crystals have crazy prices too, perhaps it would be more wise to go farming some of them instead! For this you need Mining. Arcane Crystals are found along with Thorium Ore, from Thorium nodes in high level vanilla zones.
This way you can make even more gold, perhaps thousands in a hour and more if you turn a few of them into Delicates, half into Arcanite Bars and sell rest as they are! Not to mention Thorium Bars can be expensive too, along with the other gems you can find..
TLDR
In conclusion, if items are too expensive:
- Mine Arcane Crystals
- Transmute most into Arcanite Bars
- Create Delicate Arcanite Converters
- Sell Arcane Crystals, Arcanite Bars and Delicate Arcanite Converters.
And if they are not expensive:
- Buy the materials
- Create and sell Delicate Arcanite Converters