You can always edit the percentages for whether something under the base value can actually be found, but there's plenty of reasons that something nobody is producing might be found in town. Maybe an innkeeper knows of some adventurers with a cache of scrolls they were planning to offload, or that Jeorj the Grocer accepted a bunch of scrolls as payment for trail rations and other supplies. Knowing the specific scrolls might suggest all sorts of NPCs who might have such an item stashed away.
If we are talking about magic items, you need to check for buildings that can actually produce magic items, such as an alchemist for potions and minor wondrous items.
In my 2 tables i ran with kingdom building, they needed some building capable of selling those items.
They needed a blacksmith for weapons and armors made of metal.
They needed a mage tower, library, shrine or similar for scrolls.
An alchemist, temple or church for potions.
And at least one trade shop quarter for all the various mundane items.
Now, i would simply roll a 75% chance (as per CRB) to find any mundane or magic item that the city is capable of producing and is within it's base value. If not available, try against next week, or comission it (rules for comissions can be found on ultimate ruleship from legendary games).
I'm pretty much with shadowkras (for an elf, he's quite insightful, and knows his stuff), except that I'd say wait until next kingdom turn before checking the 75% chance again (nothing wrong with a week, but a month would be more consistent).
Basically, the kingdom rules are a subsystem, but they don't modify the core rules unless explicitly stated. So your settlements have a base value which may differ from the core rules, but what that base value means does not change (75% chance to find any item below base value).
@Philip, you are correct.
But a city without those buildings means it doesn't have it available consistently. A caravan might not have a Base Value of 2,000 gp, but instead only up to 1,000 gp, and might be available only during one week each month (trade routes are actually good for this).
They might sell weapons, but you cannot ask one of the merchants to craft you a greatsword simply because he is a blacksmith, he would need a few days to spend crafting, or have it on stock.
Those merchants, without a fixed building available to them, might be using tents, improvised tools, might be working on terrible conditions, have no space to stock their goods, and so on.
Kingdom magic items availability are laid out pretty simply in the Kingdom Turn Sequence section of the Ultimate Campaign book.
Gaining Item Slots: When you construct one of these buildings, mark the appropriate boxes in the Magic Items section of the settlement's District Grid; this indicates that the settlement has gained a slot for an item of that type.
Filling Item Slots: In Step 3 of the Upkeep phase, you roll to fill vacant magic item slots in each district. Roll d% once for each district that has an open magic item slot (if the district has more than one, select one randomly). There is a 50% chance (51—100) that an appropriate magic item becomes available in that slot. This item's price cannot exceed the base value for the settlement (reroll if the item's price exceeds the settlement's base value).
Example: Jessica's settlement has a base value of 200 gp. She built an Herbalist last turn, giving the settlement 1 minor potion slot. In the Upkeep phase this turn, she rolls d% and gets a result of 62, meaning she can roll a random minor potion to fill the settlement's empty slot. She rolls on Table 15—12: Potions (Core Rulebook 478) and gets a result of 45, indicating a potion of a 1st-level spell. If she had rolled anything more valuable than the 200 gp base value for her settlement, she would have to reroll until she got an acceptable result. Once a magic item is rolled for a settlement in this way, it remains on the market until someone purchases it.
Pathfinder RPG Ultimate Equipment includes extensive random magic item tables for specific slots and price increments. These tables may be more convenient than using the magic item tables in the Core Rulebook.
Emptying Item Slots: If you are unsatisfied with a magic item generated by a settlement, there are three ways to purge an undesirable item and make its slot vacant. The first is to purchase it with your own gp, which makes it your personal property and means you may do with it what you please (use it, sell it at half price for gold, deposit it in the kingdom's Treasury during the next Income phase, use it as a reward for a local general, and so on).
The second method is to manipulate your kingdom's economy to encourage an NPC to purchase the item (such as a random adventurer passing through the settlement). During Step 3 of the Income phase, you may attempt one Economy check for each filled slot you want to empty. For every such check after the first one in a turn, your Economy decreases by 1, since these manipulations are harmful to your kingdom's economy and typically only serve to get rid of an item you consider undesirable. If the check fails, nothing happens. If the check succeeds, erase the item from that slot; you may attempt to fill the empty slot as normal in the next Upkeep phase. You do not gain any gp or BP from this sale; the money goes to the building's owner, who uses it to acquire or craft the next item.
The third way is to spend BP (1 BP = 2,000 gp) to purchase the item. If you take the item for your own use, this counts as withdrawing BP from the Treasury for your personal use (see Make Withdrawals from the Treasury). If you use the item in a way that doesn't directly benefit you or the other PCs (such as giving it to a hero of your army or donating it to a settlement as a religious or historical artifact), then purchasing it is essentially like other kingdom expenditures and does not increase Unrest or decrease Loyalty.
In short, the special buildings add additional specific magic items.
Anything other than this is houserule territory.
What is the process for players wanting to buy items from their Kingdom? If the item is below the Base Value, does it not matter if the city has the capabilities of creating it?
I have a player who wants to buy scrolls, but the city has nothing letting them create scrolls - but the current base value of the city is more than enough to purchase scrolls.
Just wondering what the right way to go about this is.
You have a player that want to buy something from someone/anyone willing to sell that something. That is different than buying something from the crafter of the something.
If the item is less than the Settlement's Base Value, check once a month to see if the item is available.
If it is not, the player can always ask some merchant if they could get the item, but that will likely increase the price. Give the merchant a percentage chance to locate the item each month or week depending on how much the player is willing to pay.
Or, the player can commission someone to Craft the item, assuming they can find someone who can do it.
The scrolls available as Minor Magic Items in a District are representative of the District's ability to supply "X" number of Minor Magic Items per month. The Magic Items available in a District are not a limit as to what exactly is available.
If you are using Legendary Games Ultimate Rulership, you can reduce the Magic Items available with Commission Edicts as you "direct" your crafters to create specific items. But that's more complicated. I'm not sure, but it might be possible to reduce the Magic Items available from nearby District's using Commission Edicts.
There is enough downtime built into King maker that the PC has the time to make a trip into Restov or even New Stetven, to buy things that are unavailable locally. or even to commission a merchant to order and collect it for them.
Alternately you could set up some story line possibilities -
1) Perhaps a mage with a questionable background is looking for a patron.
2) Or there is a man, who knows someone, who knows one who might be able to help for a price *Nudge Nudge Wink Wink say no more guvnor*
3) The Mighty Hanspur has just the scroll you need. Come take a walk with me down by the river and we can see if you are one of the chosen ...