One tip is to monitor your server on theunderminejournal.com
- it has both current and historical prices grouped by profession, by category (pets, transmog etc.). There are heat maps so you can see what time of day/week different items tend to list at what price, which can help you figure out when to buy.
For example, you might find that crafting materials get cheap over the weekend then spike on raid reset days (Tuesday). From that you can build a shopping strategy where you buy materials on Saturday morning, then craft and start posting things on Monday, Tues., Weds afternoon. This tends to work especially well for raid consumables (potions, flasks, vantus runes etc.).