If you like to level alts or otherwise farm low level instances, you're bound to find one, if not more of certain items that are a lot more valuable than they would appear to be. Most of the time people pass on these, sell them for ridiculously low prices or just vendor them, so clicking on need is highly recommended.
You should also check your AH if someone is doing the above and benefit from it.
World drop items - How to obtain?
One of the most common rare items is the manual for Strong Anti-Venom, a random world drop, mainly obtained from chests around level 30. Even though farming world drops is pointless, this is not the only valuable item a chest may contain. A good place to test your luck is Scarlet Monastery.
There are more than one wing, so the 5 instances per hour limit will not happen as fast as it would in a typical instance. There are no chests in Cathedral, but mobs also can drop valuables and that wing is the fastest in the game anyways :)
Half of the chests are locked, so bring something that can open them, like bombs or skeleton keys. Skeleton keys do not need a profession so they are often the good choice.
How valuable is valuable?
Most of the valuable items I will be listing here are worth 2-15k, sometimes more. They are mainly for collectors who want to learn every possible recipe in the game. That means you shouldn't expect these items to sell even in the first week. It may very well take even a few months to make one sell, depending on how much your are asking for it.
- If you create an interesting macro that advertises your rare loot, it will catch more eyes and sell a lot faster.
- You will be the only one selling these most of the time, so you get to pick the price!
I will create a similar gold making method containing a "junk" rare item every 1-2 weeks. Next item I will tell about is a bit more rare, but also a lot more valuable. I've sold one of them for 20k after buying it for 15g a few weeks back!
Hint: It's Purple