This has been discussed officially on multiple occasions, they won't do it for server stress reasons. The AH is basically handled by its own special server, and increasing the slot cap by even 1 for all users could increase the load by a significant percentage (Over 10% at maximum for just 1 more slot if everyone filled up all their slots at a given time. Obviously not everyone will, but the point is the load is likely to increase proportionally to the number of slots).2: Please let us add more items on AH for sale.
Letting people buy part of a stack also opens up a huge can of worms because it creates a lot more to keep track of. Prices would also need to be set per item instead of for the stack. This would be a fundamental design change that is likely not possible at this point.
However, a pro tip: The outcome of your bid is actually nearly immediate- If you back out of the menu when it says "placing bid" the result of the transaction will happen immediately and you can jump right back in to buying more items instead of waiting the 3-5 seconds or w/e it usually takes.
(it's well known that if you use... things... that can keep your gil on screen at all times, if you make a successful bid, the gil is subtracted from your total nearly the instant you hit enter- and if it's not successful, the amount of your bid is very briefly deducted and then returned. So the reality is, they make people wait a few seconds for no real reason, except possibly to slow things down a bit deliberately to limit server load)

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