Not being able to buy single items from a stack on the marketward.

[Jinko]

3. Allow the divisibility of bundles, but grant a wholesale discount to whoever buys everything or a large quantity - my offer is one of 20 items, but you can either buy 5 at price x each or you can buy 20 at price x*a each, with a < 1.

This is a very good idea.

Of course, this requires that they level their guys to 20 do the applicable story quests and defeat Ifrit with their dummy characters. that's in addition to this work of constantly managing their retainers..

RMT have no problems with doing these kinds of the things they farm for hours on end every day, a few menial quests are nothing to them.

FWIW you don't need to defeat Ifrit to get retainer access.