Fantasy Settlements - part 1: Resources

Players have a habit of treating settlements like vending machines for equipment, so let's indulge that tendency, and look at what you can get from a given size of settlement.

D&D 3rd edition gave us 8 categories of settlement: Thorp, Hamlet, Village, Small town, Large town, Small city, Large city and Metropolis. I'll use those terms, as they're as good as any.

Normal life experiences tell us that less goods and services available in a three family thorp than are available in a village of 100 families, and that even more choice will be available in the city of several thousand families.

How can we reflect this in game?

Simple cash limit
The simplest method is to apply a cash limit to each settlement category - and this is what D&D 3 did. It's a reasonable enough method, and easy to implement.
But in actual play, a straight use of these rules can lead to odd situations: a rural thorp might have several swords available to buy, but no horses - or a large port town might have magical armour for sale, but no sailing boats.

Categorised cash limits 
It seems obvious to me that we need to have differing cash limits for different goods and services.
The GM can of course make ad hoc rulings to deal with this - but I'd like to at least make some guidelines.

First, I'll establish some categories:

Trade goods
General gear
Adventuring gear
Special substances and items
General tools
Specialist tools
Clothing
Mounts and related gear
Transport
Arms and armour
General services
Expert services

...and lastly - Magical

 These reflect the categories for equipment given in the D&D rules, with only a few tweaks. They're good categories, generally.
All I've done is add "Magical" and split a couple of the D&D categories: "adventuring gear" is split into "general" and "adventuring" - because despite their equal costs, I imagine that in a normal settlement grappling hooks would be less common than flint and steel, and a portable ram would be less common than a tent.
Similarly, I've split "tools" into "General" and "Specialist", because I think it'd be harder to find a disguise kit than a healer's kit, and "Services" into "General" and "Expert" for the same sort of reasons.

That's too big a list to manage all at once, though - so, I'll gather up those categories into the following groups:

Trade
General
Specialist
Magical

Trade and Magical include Trade goods and Magical goods (obviously)
General includes General gear, General services and General tools (again, obviously),  as well as Clothing, Mounts, and Transport
Specialist includes Adventuring gear, Special substances and Specialist tools (continuing our obviousness motif), Arms and armour, and Expert services

So what to do with these groups? Assign weight to them!

Trade goods should by default, use the standard  (or base) cash limit
General goods should be at 125% of the base cash limit
Specialist goods should be at 75% of the base limit
Magical goods should be 50% of the base limit
(Round off to the nearest 5 coins)

So for a Thorp in the D&D 3rd edition rules, the cash limits would be:

Trade 50gp
General goods 65gp
Specialist goods 35gp
Magical goods 25gp

Which means that the thorp might be able to sell you 5 cows, some masterwork artisans' tools, a flask of holy water or a single 1st level spell scroll - but it doesn't have a disguise kit, a stock of potions, or anyone who can put weaponised spikes on your armour.

So this option seems to do the job I wanted.

But we should retain flexibility - rules shouldn't be a straighjacket - so these cash limits for a given settlement should also be freely be adjusted up and down as the GM sees fit.
Let's say that we can adjust the cash limit of any category in a given settlement by 1/4 of the base limit.
This lets the GM set the availability of specialist goods higher or lower, as appropriate for the setting.

Local goods for local people
Of course, that list doesn't give us a chance to deal with any local bias - our port town example would still hold if we don't split water transport out from land transport and the like.
I think the best way to deal with this is to add both a "local" and "other" descriptor to the following categories:

Services (General or Expert)
Transport
Trade goods 

The GM should also be entitled to add the local and other descriptors to any other category as appropriate.
Local goods should get a 25% boost to their cash limit - it's easier to find them in the local shops and markets.

All the work you want
Of course, the flat cash limit works fine if you're not thinking too hard about it.
If there's no need to fiddle with the cash limits for a given town or village, or if there's no time, then the base limit can apply to everything.