Is your feature request related to a problem? Please describe.
It is not really a problem but quite an inconvenience.

Describe the solution you'd like
At the moment, I can assign a single default quantity for sales. But items come in different stack limits: some have a stack size limit of 200, 20, 10 and 5.
It would be nice to assign different quantities that depends on different stack size limit; maybe at least two options, one for size 20 and another for size 200, as those are the most common in my experience.
For example, I would like to sell a quantity of 25 if that item in question has a stack limit of 200, but a quantity of 5 if the item has a stack limit of 20.

In the alpha builds there's an extending quantity settings per-category that should let you configure it how you want it:

Does that do what you wanted?

Closing, the feature appears to do what was requested and is now available in the normal Auctionator releases.

Thank you, I will test this :)