Here's a tip within a tip: Any time you're standing around a capitol city, say while in between battleground queues, go to the auction house and search for Netherweave Cloth. Buy all of the cloth that's listed for under 3g a stack, or 15s per piece.
Now, make it all into stacks of Heavy Netherweave Bandages, which vendor for 3g per 10.
See what I did there? I do this every day and make a small profit off of someone else's ignorance of the marketplace.
What did I know that they didn't? One, I knew not to get crazy undercutting prices on the Auction House. Two, I knew that bandages vendor for more than raw cloth, and how much that price is.
That's just one example of making a profit from price cutters. If you know the true market value of an item, you can often make a profit from speculation by buying and reselling underpriced goods.
It's also important to know the worth of items when you're doing any kind of grinding. Ideally, you want to grind on the same mob over and over again, so that you get the same trash loot drops, which stacks save bag space, and make for less trips to a vendor, and more grinding uptime for the items you're actually farming. However, bags do tend to fill up.
So, is that Brittle Horn worth more than that Dirt-Covered Fur? And is a stack of either worth more than the Stoppable Force? The vendor value of gray items can be significant, and if you need to clear bag space, it pays to know which grays have higher value than the others.
For the Auction House, I highly recommend the Auctioneer UI Addon. It's been one of the most popular mods forever, and there's a good reason.
For sorting out your junk loot, I like SellValue.