I've wondered about this. Mostly; why are some items duped all of the time, when others, which would sell just as well, are not?
The vial of the sands itself doesn't seem to be what people are copying, its the raw material. I see people in trade trying to sell stacks of the raw materials for 2-3k. Although this could be a limit on how many people have been able to find the recipe.
The deathchargers sell well. But all of the items from shadowmourne are BoE. Why don't we see the tabard duped just as much as the mount?
I've heard that higher-end players were duping epic red gems when they were first available. I don't know if this is true or if it was just rumors.
I've looked up the basic method to duping. But I cannot seem to figure out why we don't see all kinds of cheap items. On a high-pop server (like Illidan) you would think you could find all different mats/enchants/mounts/etc. for dirt cheap, albeit illegitimately copied.
As a note to people asking about purchasing duped items. If you buy it from the AH, the worst that could happen would be the mount would be taken away from you without refund. Although because it is a 'spell' and not an item, Blizzard might have to do this on a case-by-case basis, which makes it unlikely. It is also something you could have been ignorant to. I purchased a Vial for 10k before I knew people were duping items, because I thought someone typed the price incorrectly.
If you trade some dude from trade chat 5k for 6 deathchargers, that might get you in trouble.