The transmog market is pretty strange, and I have made tens of thousands of gold off of it.
The method is easy. Find a good snatch list for green items. Run that list. Pick up the cheap ones and repost them at a significant mark-up.
Personally, I put them up for 1200 gold each.
Most people level by questing or doing dungeons these days. They either get their gear from those activities or have heirlooms. This has made a nice opportunity to sell these items.
This morning, I woke up to three sold greens at 1200 gold each. I probably bought each piece for 10 gold or less.
One thing that I have noticed is that the most visible pieces sell the best. This includes shoulders and helms.
I have been thinking about grabbing all of the Imperial Plate Recipes to make the Stormwind guard sets. I think I could see those for a very nice little profit.