I have always wanted more slots to sell things...however, I have thought about this in the past, and the unintended consequences of such a move.
Price crashes. It sounds miniscule, but simply adding an 8th slot does change how consumables are listed. If 30 people sell say reraise earrings and post 3 every day (90), they currently use 3/7 (43%) of their AH capacity. Give them an 8th slot and they add one, to make 4/8 (50%) of their AH capacity. They still have 4 slots free, no change to them, but this adds 25% to the market. The price drops on RR earrings.
However...the supply on Mythril ingots/ores does NOT change, as farming them is not exactly a huge moneymaker compared to other methods these days. So goldsmiths start to lose money on making reraise earrings. So they stop making them. Fewer Goldsmiths make Reraise earrings, and so (and here's the key) fewer goldsmiths buy mythril. So fewer people farm it. Prices spike on the earrings, but the overall supply on mythril never fully recovers. And your reraise earrings...well, they never totally come back.
It sounds totally conspiracy theory I know, but look what Abyssea has done to MANY crafting materials. Check the AH for the prices of many previously expensive crafting materials that are now dirt cheap, and others that used to be commonly farmed items that are not ridiculously expensive. I'll show you clothcraft as an example (as it is my craft, and I'm most familiar with it):
Shining cloth: http://www.ffxiah.com/item/831/shining-cloth
over 200k back in the days where Kirin was the only source. Now, sales rate is over 3/day, unheard of 5 months ago.
Silk Cloth:
http://www.ffxiah.com/item/829/silk-cloth/?stack=1
Stack price. Less than 2 months ago, 50k a stack. A farmable item was silk thread, with cloth being a staple on the AH. Now, spiked in price to amazing levels. The nature of the game that I mention above.
I don't complain, it's the market at work. And if you add slots, you increase the volume of many things that will cause major fluctuation in the markets,
especially consumables. Just think of all aspects before you just think of being able to slap up all those wyvern skins.