Refer to them as idiots as in being 'dumb' is something I wouldn't do. Plenty of theories out there that prove that deep undercutting works.
For example yesterday I snatched 37 inferno ruby at alliance side for 40g each. I play horde and alliance on my realm is about 3 or 4 to 1 horde. Int Inferno ruby sells for 115g on horde, about 100g on alliance. Usually I sell about 2 of every red cut a day, it's not something I'm very active at. The 10 inferno's I get everyday come from flipping obsidium ore into enchanting mats, which is my goal of prospecting ore rahter than the gems. So rather than trying to sell the 37 inferno's at max price I listed all 37 cuts at 85 gold each at once, on a toon that never sells jc stuff. They were sold withing the hour at prime time, to a guy who most likely bought them to resell. I don't care about the 30*37 gold I've missed out on just because it was an extra snatch. Having to sell them over time means more work involved and price decrease risk. Not to mention that there is a possibility that I snatch more over the next day, which would cause me being stuck with a huge stock. Hence why I go for the quick profit.
As for price resets: I only do that for markets with little supply, 'never ending markets' or where the time is in my advantage.
Example 1: glyphs: glyphs are easy to make, a large part is pretty cheap to craft and very beneficial. I get my ethereal inks from alliance side, which makes my crafting cost 21g per glyph, those who buy horde herbs have a crafting cost of 80-90g. My sell price for these glyphs is 145g. Whenever someone enters the market and undercuts me I sell them for 75g. Everyone who tries to buy me out is just a fool. I keep mass supplying them for lower and lower, eventually pushing them out. Whenever he decides to get out of ethereal glyph market and dumps them on the AH, I buy him out and start back to list at 145. The main glyph guys stay away cause they know it's a lost battle. This is not true for inks that are available at a way more reasonable price, I just undercut for 1c.
Example 2: belt buckles are not listed en masse on horde side. There is not a constant supply from the same people. Yesterday they were up to 375 gold. And everyone needs belt buckles. Sometimes, typically near end of the week before reset they can go down as low as 220 gold. If there are only 3-5 buckles up at that price, I just buy them out and relist at 375. They sell anyways. The other guys who sell buckles basically do the same with me to reset the price. Eventually we all gain with it by forcing a price reset.
Example 3: saronite ore. Prior to cata I stocked 9 gbank tabs full with saronite ore. A pretty safe long term investment since if the price wouldn't go up as I expected, I could vendor them and run breakeven. Have about 4 gbank tabs left now and price is 8g a bar (I only bought ore below 0.70g means I make a 6.6g gain per bar). There is very little supply. And people who level professions need tons of them. Usually I list about 100 bars once a day. But if none is leveling professions, other people list a few bars a day for less gold, the next day same, etc. I've seen price going down to 4.5g where there was only like 50 bars listed. The it's very safe for me to buy them all out and relist. I'll make a profit anyways, even in the long run.
Edit: 2 other dangers of resetting the price is a/ people automatically list at market price rather than 1c undercut or b/ people not looking at prices (like me for many stuff) and just let the addon put it up on AH en masse at a predefined price. Thus screwing you over or forcing another buy out.
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