Why you need extra retainers

[Aeyis]

Doesn't seem so unusual to me. There are plenty of reasons why someone would buy a bunch of HQ mats.

Those are very large amounts tho. High level crafters dont actually need or want low level HQ mats (because they are more expensive), unless its needed for a specific 2* (for use in macros, if the price is similar enough to the NQ) or higher.
Low level crafters dont have any use for HQ items of those amounts.

As a result on my server those kind of items typically sell in stacks of up to 10, while rarely in increments of 20-40.

Also... when people complain about not enough retainers, it's because they lack inventory space. It has nothing to do with Gil.

Pretty much, yeah. Altho + 1/2 retainers can help with selling if you go through large volumes of different sizes.