Everyone loves undercutters. People just hate them when they're selling the same thing. How did I make money? I mostly sold wild onions, silk thread, and insect paste balls. I got lucky with Spiny Spippy a few times. I got the Bombking's Bombring, the Cruel Scythe from one of the Dopplegangers in Horototo. I got lucky in a gamble with four tree saplings, and got 100k from a Dark Ore (which was dropping in price at the time, but I did ok). I did the Aspir, Warp and Drain quests and sold them to invest in culinary to make my money back. Doing those things let me equip my white mage decently - +1 seers and some misc level 34-35 stuff. However, I'm a summoner now. I sold all of my white mage stuff. That's how I got my 500k or so. Not everyone with spare change bought it from IGE, you know.
Undercutters are great when you're buying, and price jackers are great when you're selling. You have to play the system. I didn't buy Astrals when they were 250,000 each. I waited for everyone to cash in on easy gil and flood the market, then got mine for 100k less. I don't sell my stuff when there are 100 in stock and the price drops lower than an Elvaan's INT. If your stuff isn't selling for the price you want, just wait. Eventually the excess stuff will be bought, and you can name your price. If you want the AH to cater to your personal whims - dropping when you need to buy, and raising when you need to sell, you're just out of luck. Patience is a virtue.