I do a lot of business at the AH, and as someone who has managed to make quite a bit of money over time, I have a few suggestions to share with new players that I hope are helpful. First, a preface: I'm a casual player, not a hardcore crafter, and as such, most (but not all) of my business has been in materials. I personally recommend materials as a very cost-effective--no extra ce spent, for example--alternative to hunting recipes/crafting vast quantities of an item in hopes of a desirable UV to auction, but for those of you who ARE looking to craft for UVs in hopes of getting a winner, most of this still applies.
Suggestion #1: Anything you might auction is only worth what someone else is willing to pay for it. It doesn't really matter what *rating an item/piece of gear has if no one is interested in buying it. If you're looking to post something for auction, run a search first to see how many are out there, and if anyone is bidding/buying.
Suggestion #2: Set your pricing to the items that are actually selling/being bid upon. You can ask for as many k as you'd like for an item, but even if there is strong demand for what you're auctioning, people will still bid/buy elsewhere if your price is too high.
Suggestion #3: Whenever possible, aim your price for a BUY instead of a bid. You may not get as big a score on a single auction, but you'll sell significantly more, and much faster. To check for a good buy price, search for the item and set the results for buy price. Once your auction is posted, if the item you're selling is in the top 5-10 for the amount of time you set, you should be in good shape.
Suggestion #4 (specifically for selling materials): Price/group items based on what they're used for. Check the wiki--how many recipes use the item? How many of the item do the recipes use? Example: if someone needs exactly 4 of an item to craft a specific, popular piece of gear, and you have 4 of said item, see what happens if you auction your items in a group of 4 instead of individually--if you had the cr, what would you do: enter a bunch of auctions for single items in hopes of winning each, or bid/buy the lot that already has the number you need, right?
Suggestion #5: DON'T SPAM THE AUCTION HOUSE. Trying to auction a ton of individual items at the same price isn't going to make your auctions more desirable: it only takes one person posting something at a better bid/buy price to make your auctions nothing but noise. Unless you're really strapped for cr, try only posting one auction of a specific item at a time while following suggestions 1-4.
And lastly, Suggestion #6: The market changes, and you need to be sensitive to those changes. The recent spike in the price of sword stones is the perfect example: a temporary surge in demand created a temporary surge in price, and what is only a 2* item--and pretty easy for tier2 players to get their hands on by going through a bunch of mech levels--was suddenly expensive. When a bunch of people rushed the AH with sword stones in hand, trying to make a buck, the increased supply pushed the price back down. Happens all the time. Read the forums, check the availability and pricing of items being offered at the AH, and try to catch the up-swing in price while following suggestions 1-5.
Hope this helps. I'll answer questions as best I can.
1) I presume the reason you're posting this is to keep people from putting up auctions for items that there are already a ton of, but range many 100% difference in price from top to bottom.
Its possible the item was undercut after it was posted. It's possible they put a low bid with a higher buy with the intention of trying to get people to bid it up and if not buy it out. Patient people can make a lot people money with proper bids than with low buyouts, just depends on how patient you are.
2) Pretty much the same as the first. Auction things that people are buying...well since you have no way of seeing past sales you have no way of verifying this other than anecdotal evidence. Not really much of a suggestion
3) This is totally matter of personal preference. Buyers should be aiming for buy prices, but with the 5 min reset on every bid it makes winning a bid a pain in the ass. Since it could be a bot on the other end that's bidding against you.
My suggestion is either decide to play for buyout game or play a bid game, but you can't easily do both. Set your Buy price for what you're willing to accept for it, and set your bid price for the absolute minimum you are willing to accept. Then if people want to bid for it they can, but if they buy it you get what you want for it.
4) People say this all the time, but my experience is posting single auctions is just as lucrative as posting groups. People don't like it because its spammy, but to be honest there have been plenty of times when I only needed 1 or 2 of an item and not a whole stack. Yes I could buy the stack and resell the rest, but its also just as easy to buy individual items.
If the item is worth less than 100CR then I say post in bigger chunks, but anything over that I just post singles. Mainly because I wrote a nice macro to auto post the auctions and its easier to verify that a single item is up for posting than multiples.
5) See answer to question 4. Meh there is no limit to the number of auctions, there is no increased cost for the auctions, and it gives people the most options for what quantity to buy. Annoying for the people that want to buy quantity, but if I was looking to sell in quantity I'd sell in trade.
6) Of course the market changes and what you described isn't a failure of anyone doing anything, but stealing your business. Someone was willing to accept less for the item you are selling and they got the sale. You can say that they should have gotten more for the item, but that's you crying over lost sales not them losing lost profits. They made their profit and are already on to the next profit. You on the other hand are standing there with unsold items. There is no one correct price and lowering your price to beat the competition is a strategic move that may or may not work. That's the gamble that you are taking by trying to maximize profits rather than dealing in volume.
So yeah sell what you want for the price you are willing to accept for it. Happily undercut your competition and watch them rage while your pockets are full to the brim with Crowns.