This might be\probably is a repost, but I didn't find it when I searched so here goes...
This can be done with anything that is commonly sold in stacks of 20, I will be using silk as an example here.
On the Ah, say silk is going for 1g a stack and there are about 10 stacks.
for the exact same price all of the other is going, put up at least 2 auctions.
One with 20 for the same price all the other silk is going for, and another with 2 silk for that price as well.
It can also be done vice versa where you have a stack of 20 for 10g rather then a stack of 2 for 1g.
How this works is when people are buying a lot of the silk, they will not pay attention to the amount, and just keep on buying the stacks.
it is more likely the work if it is 2 for 1g, but the other way will get you way more profit.
If the average price changes, it will stand out, and it won't work, that is why it is best to have one or two of your own with it since then it still has a chance to work
Again, sorry if it is a repost (probably is)