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Saeviomagy
So, NPC's can never do anything that the book doesn't explicitly have rules for?

But if you see the book as a set of basic guidelines or examples of how things can be done then you have no problem deciding how you want to handle it as a DM.
Back when I first played BECMI (and then 2e) with my friends, we were very fast and loose with the rules, at least as far as NPCs were concerned. For example, the DMs in our group often had NPC spell-casters simply conjure up whatever effects were apposite. If drama demanded that the NPC conjured up a brick wall across the corridor, they did. In other words, we went with what was narratively or dramatically desirable; and NPCs didn't need to have the same type of magic as PCs; they didn't operate under the same (or any) rules.
Then, I remember buying 2e materials and seeing the exact spells used by the NPCs laid out within the game rules for PCs, and thinking, basically, "WTF". "This NPC can do this, this and this, and is constrained to that, and nothing else, because that is The Rules". Suddenly, the game felt much smaller because everything was constrained by The Rules. I'm not saying that we were doing it right, and other people wrong, but it does show how you can have a group of DMs running a network of perfectly enjoyable games without even knowing that anyone ever used The Rules to govern every facet of NPC behaviour.
Nowadays, I'm somewhere in between; I often plan out the spell lists for NPCs according to PC class rules, but I will improvise during play, giving spellcasters more spell slots, or fewer, on the fly, if it makes an encounter more fun. I really don't want everything in my campaign to be constrained by The Rules.
I realise that not everyone wants to play the same way, but I do think that somewhere along the line, too many players have got too fixated on The Rules, instead of having a shared story-telling experience. Looking at NPC businesses, it takes about 10 minutes (at most) to Google a few articles on medieval economies and make a few decisions.
That's your "rules" for how NPCs earn money - just decide how your campaign economy works, and then pluck some numbers from the air. Who cares if you can't stat it out an NPC's income in double-entry book-keeping form, and then publish the fully audited accounts in triplicate? Just make a decision, and then spend the time saved creating adventures and the actual meaningful parts of your world, like cultures, antagonists and allies.
Edit: and this is why the 5e DMG is so great. It contains tonnes of food for thought when designing a game - lots of fuel for the imagination - without overloading the game (and IMO wasting the DM's time) with "rules for this, rules for that, rules for the other".
Edit 2 (from the Pointless Musing Department): ... all of which is psychologically interesting; in real life, I'm someone who tends to follow the rules. If I play a board game I'm a stickler for the rules. When I play RPGs, however, as far as I'm concerned the rules are there as a guideline, but you really don't need them that much. I wonder if there's any correlation between people's real-world attitudes to rules and rules in RPGs?