My character GUI looks just like before. Is there supposed to be some hot key to activate it. Why is there not one scrap of information on how to use this mod?

if the recipes haven't been calculated it won't show up.
there should be a small window at the top displaying the calculation progress.

There was no such thing, but the buttons did eventually show up. I found them after I came back from lunch. It would nice to tell people about this in the description.

Next question: is there any way to specify how many I want to buy or sell? Thanks!

if the recipes haven't been calculated it won't show up.
there should be a small window at the top displaying the calculation progress.

At the top of what? The main landscape display, the map, the mini-map, the character GUI, a chest GUI, or what. It has been over 20 minutes, in a heavily-modded Factorio 0.17.72, and I can't find any sign of the Market mod.

Mods list:
almost all AAI except AAI Industries
Aircraft
Afraid of the Dark
BeastFinder
Bob's: enemies, greenhouse, inserters, warfare, ores, MCI, library, logistics, plates, tech
Bottleneck
Factorissimo 2
Fill4Me
Helicopters
Nanobots
SqueakThrough
Angel's Warehouses
Even Distribution
More Minimap Autohide
QoL Research (aidiakapi's)
stdlib

I noticed the calculation display at the top of the landscape upon loading a saved game. Still no buttons on the character inventory. Is there some hot key or command to activate it?

I've seen the green and red buttons appear for a while, and disappear a bit later in the same session.