Potionomics

Finite magical items mainly include wands, scrolls, arrows, and potions. Potions are generally the most widely manufactured and traded because they are so easy to be used.

Economics of Finite Use Magical Items

Scarterran haggling scene by Zeta Gardner

Potions and other finite magical items are essentially very high end commodities and can be bought and sold as such. Many larger market places have some potions circulating, but few have lots of of potions.
Healing potions and other ●●● magic potions typically trade between 50 and 70 silver pieces each. That is roughly what an unskilled laborer makes in a year.
Fast Movement and and other ● level magic potions typically trade between 5 and 6 silver pieces each. That is roughly what an unskilled laborer makes in a month.
Adventurers often carry many of these potions at at time. It is expensive, but a single potion can sometimes be the difference between victory and defeat, so for many, it is worth it.

It may seem illogical that a world can exist where a common person's annual income can be stored in a tiny vial and consumed in a few seconds, but epic adventures and war in general are expensive, always has been and always will be.

To use a real world example, there are missles and smart bombs that well cost well over a million dollars for something designed to explode and never be used again. It is not uncommon to see the targets destroyed be far less expensive than the ammo being used to destroy them.
Advanced warfare has always been expensive as was in the 20th century, the 19th century, all the way back to Ancient times.
The reagents based economy in Scarterra is based on nobles, high priests, and other princes and potentates want to collect and horde reagents so they can collect and horde potions and then use these stockpiles to win critical battles, much as a modern military stock pile smart munitions. Some potions and finite magical items even explode making the metaphor that much more on the nose.

by Me with Midjourney

Many princes seek to legally restrict who can purchase or own a potion or wand and who can or cannot, but at the end of the day, the biggest factor limiting who can or cannot own a potion is cost.

by Eron12 with Heroforge

Sir Casmir, Grand Marshal of Fumaya

"Magical healing is especially valuable. They can save the dying so they can fight another day. Full potions can let a moderately wounded person and let them go back into the fight almost immediately.
Even if there is not enough magical healing for everyone, the mere possibility of receiving magical healing encourages men to fight more bravely.
Spell augmentations in potion form are handy to make soldiers fight better to avoid being wounded in the first place and take out the enemy quickly. The right magic can make mediocre soldiers fight like elite soldiers, but if you have limited augmentation magic to go around, it is best to give it your best soldiers, those who are already elite. The right magic can let a skilled warrior fight like a hero of legend."

Acquiring finite magic items in game

If a player character adventurer wants to buy a potion, and that PC is viewed as an ally of the nation (or tribe or religious faction, etc), that adventurer can probably buy potions and other finite magical items at cost or with a very small markup to pay the living expenses of the potion crafter.
Many potion crafters accept reagents in place of coin. Some prefer accepting reagents over coin. If a potioneer uses 50 drams of reagents to make a potion and sells the potion for 60 drams of reagents, he can replenish his stock and come out ahead.
If the PC is viewed as neutral to powers that be, they can probably buy potions at a modest markup, assuming the items are available. An PC that is viewed as an enemy of the land she is in is going to have to buy these things from the magical black market at a large markup.
Whether legal or illegal, potions and other finite magical items scale in price greatly as the base magical power increases because making ●●●● and ●●●●● dot potions and scrolls is highly prone to error when crafted by any but a true master. On the other hand, ● and ●● potions tend to have very small markups.

Popular Divine Potions

It is relatively easy to find finite magic items for sale involving Healing, Purification, Augmentation, and Divination because these magical spheres meet such consistent demand those who can manufacture these items often choose to stockpile them for sale or trade.
Outside of these divine lore, it is hardly impossible to purchase them but one usually has to ask someone to make them because few horde these potions in storage.
Potions and scrolls of Healing ●●● are in high demand with soldiers and adventurers. Most priesthoods that have the capacity to do so, will stock pile them to sell or trade.
Healing ● is usually built into a staff or wand with multiple charges. This lets a medic to stabilize many severely wounded people during triage situations.
Few bother with magical items imbued with Healing Healing ●● because these wounds are rarely life threatening and can usually be healed with simple time and rest.
Purification magic items are widely sought out at various levels, equally likely to be built into a potion or wand.
Augmentation is very popular to put in potions or charged magical items. Augmentation ● potions are so prevalent in society at large that precautions are put in place at tournaments and athletic contests to prevent performance enhancing augmentation.
Divination potions are widely sought after. A lot of people involved in security like to have ready access to Divination ●, it won't help you fix the magical problem, but it will least tell you if the problem you are facing is magical or not.
Some diplomats or adventurers have an emergency potion or charged item of Divination ●● if they find themselves stuck in a situations where they cannot communicate with who is in front of them.
After facing hostile illusion magic, most swear they never want to do so again. Divination ●●● magic items gives a counter to this for those who can afford it.

Popular Arcane Potions

In the school of Transmutation, Potions of Fast Movement and Fly are commonly sought after by those wishes to make a quick getaway or to prevent a quick getaway. Many Transmuters capable of making potions or wands of these keep a small stockpile to sell or trade.
In the school of Abjuration, Protection, Shield, and Dispel Magic items are in high demand resulting in many Abjurers stockpiling them for sell or trade.
In the school of Illusion, potions of Blur, Disguise Self, and Invisibility are fairly common.
In the school of Enchantment, potions of Heroism are widely sought after.
Other potions are certainly possible but usually have to be custom ordered.