Sometimes there's easy gold to be made if you take advantage of rich/inpatient/clueless people. You may have noticed that everyday someone lists extremely cheap items in the Auction House. Today I suggest you to buy those and relist them for, lets say.. 20x higher prices. Oh yes, they will sell!Do you want more gold tips? Don't forget to share!
If you're poor, I wouldn't suggest you to buy 20g-> stacks. Instead, look for 2g stacks! Selling these will often take days.
Here's a list of herbs that you may see someone sell for just a few gold pieces, yet when a scribe or alchemist needs them to level their profession, they are ready to pay even 100g a stack (Except Silverleaf and Peacebloom), 150g in case of TBC herbs! You may want to test your limits too, my record is Nightmare Vine sold for 600g/stack!
Someone undercutting your prices? Worry not. If their prices are super cheap, buy them. If just a few gold pieces lower than you, I suggest you to be patient. Alchemists and scribes need a ton of herbs. They will buy yours too.
Classic flowers - Top price varies between 20g-100g/stack:
Mageroyal (You may want to wait until Children's Week!)
TBC flowers - May go for even 150g/stack:
Nightmare Vine (Slow seller but great profit!)
Mana Thistle (Slow seller but great profit!)
Netherbloom (Slow seller but great profit!)
There may be more, but these are the ones that people will sell for ridiculously low prices usually and also will be sold fast. I wouldn't bother too much with WoTLK or Cata herbs, they are too easily available and people will be undercutting you a lot!
There are lots of other trade goods that are worth even more, so I will post them next.
So what to buy?
Personally I don't buy stuff that has many sellers and costs more than 20g a stack. However if there's only one seller and they have listed the goodies for 50g a stack, I will buy them if they are of a higher tier and in great demand.
If you're not yet using Auctioneer addon or something similar, don't wait any longer!