Spare Crafting Materials
Check spare materials before selling them to a vendor. Another player may need them even when you don’t.
Cloth, herbs, ore and leather are good categories to inspect. Meat, fish, gems and other ingredients also deserve a check when they have a crafting use.
Treat them as items to investigate, rather than guaranteed big sellers. Keep what you need and compare the value of the rest.
Tradeable Equipment
Check unbound equipment, particularly pieces with useful stats or an interesting effect.
Binds when equipped means equipping an item can make it bound. If you might sell it, check its current binding status and avoid equipping it just to try it out.
Green equipment is worth inspecting, but its colour alone doesn’t make it valuable. The same applies to blue and purple items: somebody still needs a reason to buy them.
Recipes
Tradeable recipes you already know, or cannot use, are worth checking.
Before learning one, decide whether you actually want it for your profession. Before selling one, make sure you won’t need it for a craft you have planned.
Crafted Items
If you want to craft items for sale, test a small batch first.
Compare the value of the ingredients with your expected return after auction fees. Materials you gathered yourself still have a value. You could have sold them instead.
For example, if the materials would sell for more than the finished item, crafting it may help your profession progress, but it isn’t necessarily making you money.