Generally, I think AI improvements to neutral (and maybe hostile, for that matter) monsters would be a good idea and add some immersion. There's another thread somewhere that talks about different possible behaviours that I can't find to reference at the moment so I'm not sure if this is covered there or not.

This is a pretty exploitable mechanic though, even if the monsters are able only able to buy "cheap" items (particularly depending on how cheap), since you can, and almost invariably will, kill the customers to steal the items that they're buying and selling from the shop.

What if this was part of a bigger idea, similar to the dungeon features ticket? Similar to how the new unique miniboss enemies work where you get a special message when you find them. 10% of the time a shop enters your view, it could have an event with an NPC leaving the shop either legally ("You see the Outlaw leave the shop with a shiny new Scroll of Identify") or illegally ("You see an Outlaw attempting to sneak out of a shop.... He's caught!" and non-Hostile thugs are summoned by the Shopkeeper). There could be other random events... maybe if you find a deep forge, there's a small chance of "You see a Chaos Dwarf mumbling to himself, attempting to forge an axe. Suddenly, there's an explosion!", and a WMoPC appears and drops a corrupting mithril axe. Or a lawful vs evil creature fight, and if you help kill one you, the winner will give you a reward (or nothing if you stand and watch).

Per the original idea: Would this work in all shops or just dungeon shops? It could be annoying if dwarfs kept buying all my potions of water in Dwarftown, especially if I have no idea which dwarves have items or which items they have. Are there any "cheap" items outside of a few shops with rocks and rations and stuff?

Aren't you about Too much effort for something that only increases immersion by a slight bit? :)
Or you're about I was a little baffled at what's going on so I followed them without intervening?

Yep, exploitable but only a bit. Usually in mid-game buying stuff you really need isn't a challenge already. But yes, depends on "how cheap". Any ideas? Unfortunately I never paid much attention on the prices?

I meant all the shops. Villages are exactly that places where PC shouldn't be the lonely customer.
I thought about water problem as well. Just decrease water rarity and increase prices. Take into account that dwarves will be selling items from time to time. Or there can be exclusions based on item types. If it's "too much effort", taking into account how much effort is spent on ADOM now btw :(, I'd rather accept decreased water rarity in the shops and still would like to see this implemented.

I lol'ed.

I guess it seems to me that dynamic shop inventory (old items disappearing in addition to new ones appearing) needs to be implemented. Even if it only changed when the pc left the level without there being any visible customers, it would add a lot to the game.

Yes, it makes the game harder if you don't know what you want is always going to be there, but if you have a more rapid turnover in addition to items being bought by other agents, it makes things much more exciting. I mean, not knowing if an item is going to be there when you come back for it makes the process of gathering gold to buy it so much more exciting.

In general, I would also like to see higher priced but more useful inventory in shops. Money is so low value right now, even if items in Waldenbrook's shop were just sort of good instead of being lousy, it would add a lot of fun. We really need his inventory to scale with progression through the game at least somewhat.

If it's in a town, there are already penalties from committing crimes (people going hostile, alignment drop, cursing, muscular dwarves, etc.), killing someone for a cheap item would just be another crime and not the worst.

If it's in a dungeon, who cares, you already get loot from killing monsters. So if you can get a cheap shop item from killing a monster this hardly seems imbalanced.