Undercutters don't bother me, cause often times I find myself doing the same thing. What I consider greedy are price-gougers who charge 50-100% increase on the item in a single transaction. I wanted an item on the AH one time, and the item was selling for 20k in nearly all of the last 10 transactions, and it's a fairly decent item, about 5-10 transactions a day, so prices were pretty stable. I needed it really badly for an event later in the day, but there was only 1 on the AH. I bid up to 30k before I refused to go any higher. I don't mind bidding higher then usual AH price, I was willing to raise the price by 50%, a substantial boost from the last transaction, but I will only go so far.
But I did just wait a few hours before checking the AH again, and another person put an item up for regular AH price, and I quickly bought it. I went back the next day and looked at the AH prices, and saw that the item I was previously bidding for ended up going for 40k, a 100% increase in price, all in 1 transaction.
The next two transactions after that were both 20k.
The concept of supply and demand is all what the AH is about, but unless something game-changing happens, there shouldn't be a 100% increase on any item in a single transaction, that's the definition of greedy. And unfortunately, someone's usually desperate enough to buy the price-gouged item, and in the end the price never changes except for that one transaction, where someone was greedy and took advantage of the desperate person.