it makes no sense that prisoners give you tons of gold for a painting that is bigger than the rotten tent they live in.
This too.
NPCs should not be interested in buying everything that you throw at them. You can sell broken bottles to armorers, weapons to herbalists and huge paintings to peasants/prisoners....
Funnily enough no NPC would buy any of the books (just normal books) that I found... unless I put them in a container and sold it together with its contents.

Leveling up is not very exciting at the moment. I have a bunch of unspent skill points because there aren't any interesting abilities I can readily purchase anyway.
The reactivity of the game is still pretty low, as it doesn't really react to some gameplay choices, e.g. you can kill all of the soldiers in the fort and essentially free the prisoners - nobody cares. You can murder all of the prisoners in the fort - the game doesn't care.
You should definitely be able to do the latter, a "psychopath playthrough" could be interesting I guess, but the game and your companions should react accordingly. Fallout for example handled this much better.