Maybe it's "Authored" vs. "Emergent"

[Cheesegear]

Why would you need a fully fleshed out NPC with backstory and family to buy a donkey?

*Points to the thread*

Is it even remotely likely PC2 cares about Jenkins?

That depends on the DM's characterisation of Old Man Jenkins. Hell, his three dead sons could make an appearance later. Published WotC modules have backstories for everyone, including shopkeepers. Literally every NPC the players ever meet is an opportunity for roleplaying. If the players engage, keep bringing them back. If they don't...Get rid of that NPC and never show them again. It's not rocket science.

So the answers would be more like
a) Ok, a donkey costs (looks up pricetable).
b) No donkey for sale atm in this hamlet. People point you to the townmarket 8 hours away.
c) Donkeys are rare here and cost double. If you remember the setting supplement, the common beasts of burden around are thorogens which you could buy for X.

The tone of this thread suggests that A and C are non-options if you're a 'good DM'.