How should I balance having Magic Shops in my world?

Question

I want to give my world the presence of magic shops that can be used by players.

From this other question:

Default 5e assumes that "magic shops" pretty much won't exist, and even goes on to suggest that potions and the like be bought at alchemist's shops instead.

From the DMG, page 135:

Unless you decide your campaign works otherwise, most magic items are so rare that they aren't available for purchase.

In order to correctly judge the question I'm also including the structure for the shop magical items and the relationship with how much the players "earn" per session of role play.

On shop availability

I plan to have a structure where magical shops follow the rules in this magical shop generator (in items available, not prices):

I'll use that as the base for generating shops and then tweak it a little bit. Idea being:

• Villages don't get any magic items, small towns and up start getting increasingly more and more magic items available for sale.

On pricing structure

I plan to model it so they will need to spend the profits of several sessions to be able to buy it. For example, I award GP to a rate between 50 and 150gp per level per session (roughly), which means a group of level 3 players will get between 150 and 450 gp per session (obviously that changes but as a median).

That means a +1 dagger will be 500gp (avg: 5 sessions per level), the price of a +1 martial item will be 1000gp (10 sessions per level) while the price of a +2 dagger will be 5000gp (50 sessions per level) and the price of a +2 martial item will be 10000gp (100 sessions per level).

That means it will still cost a level 10 character 10 sessions to get enough money to buy a +2 weapon.

Note: The base price of 1000 gp for a +1 martial weapon equates the value on the Sane Magical Prices while the price for a +2 does not. Based on that inequality you can calculate the relative adjustment that needs to be made for any item listed. For example, the Armor of Resistance is quoted at 6000 gp, so on my interpretation:

\$4000 \rightarrow 10000\$ \$6000 \rightarrow x\$

\$x = 60000/4 = 15000 GP\$

On selling items

Items will be able to be sold for 10-50% of their listed value based on an ability check against the merchant DC (which is based mostly on how big the city is and other role play factors).

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With that in mind, is it enough to just adjust CR on the encounters to account for players being able to obtain magical items through shops or is there anything else I should take into account for including magical shops in my world?

On frame challenging

I'm set on having my world have magic items available for sale on shops around the world. Obviously, the bigger the shop the more available and rare (and expensive) objects found in it will be, but I think subtracting the ability to buy magical items basically removes all value from looting money the players may have. Any frame challenge should contain a way to make my players consider money as something useful and precious... I know my players and they, like most of us I think, like their money to buy magical things so they can use it to smash monsters... that simple.

As noted on the comments by @Jack, this question handles the "money" economy. If you'd like to challenge that question can be used as reference or complement to the answer.

Answer 1

The same way you balance them without

First, in a normal game PCs will have magic items and the DMG specifically calls out that you don’t adjust encounters because of that. Potent magic items are inherently limited by the action economy - if you use one you aren’t doing something else. Passive items fall within the bounded accuracy design philosophy.

Second, you still control how many and which magic items are in the party’s hands. You choose the items in the shop or the treasure hoards and you set the price and you control the flow of money to the players. There is functionally no difference.

Third, CR is only a rough guide to how difficult encounters will be anyway.

Answer 2

Consider worldbuilding aspects of encounter-design.

This depends on what kind of world you are running the world in which the game takes places is affected by access to magical items for money. GMs have divergent opinions on whether a world has to be alive containing economic-, taxonomic- and ecological systems that are internally consistent.

I try to DM a world that has internal consistency if I have an island with two dragons and their hoards, then there are two dragon encounters and limited resources overall.

If a village has a bandit problem and that problem is resolved, then there won't be a bandit encounter on the road one day later (most of the time, sometimes players travel roads).

For the mechanical impact of magic-weapons read What is Challenge Rating?

Let me pose you questions to its answers I'm aware of in a world that I DM for the reason of balancing and designing combat-encounters when I have magical shops (that sell magical items):

Who creates these items and what would they do if they wouldn't create these items?

Do guards, mercenaries, and caravans have enough money to afford magic-weapons (are they equipped with magical weapons and items)?

What kind of defences does a village, town, or city require to stay settled?

How large is the zone of safety established by settlements (are there zones of safety)?

How do the magical items shop owners spend their money?

If you answer these questions with these are only options for players and NPCs won't be able to afford these things or merely don't have them because internal consistency is less important in your world, then make sure to balance the loot based on general magical items table progression in the DMG chapter on Treasure (DMG 133-226, Other Rewards 227-232) for details and use the tables and information provided by Xanathar's Guide to Everything on Awarding Magical Items (135-136) to ensure that your players won't gain too many major magic items which would make them over-equipped and invalidate the CRs that are balanced on creating appropriate Encounters (DMG 81-86) compare to random encounters which do not have to be balanced to be level appropriate (DMG 87). Structure your pricing accordingly or use the table for Magic Item Price (XGtE 126).

I think according to these rough outlines, moduled from years of experience of building homebrew-worlds:

The abundance of magical items creates a generally safer world with fewer adventures, fewer dangers, and more save heavens. Higher CR creatures also adapt to this world and will assume that anyone may have access to magical weapons.

A group of low CR monsters is much less likely to harass travelling groups that are guarded by people who have access to magical items (or they are exterminated quicker if considered a threat).

Intelligent creatures that have resistances or immunities from nonmagical attacks are going to be more cautious and better prepared in a world where magical items are more accessible. They are also likely to be less common.

Another aspect of balancing encounters is: What impact does spending your gold on magical items have on social-encounters? Are you expecting them to spend their gold on magical items, or do you want to give them alternatives to what gold is for normally?

The worldbuilding issues caused by magical items shops (and all that annoying additional bookkeeping to track resources) are the reason why instead of magical item shops I now exclusively run auction houses where you can sometimes but not always buy magical items and sell, auction or hire someone to auction your magical items for you and all "magic shops" are for supplements, books, information etc.

Money is a means to an end. So why would you want to auto-solve loot instead of gaining that by adventuring or creating adventures like sabotages of auctions where you can use the money to make things possible instead of instantly solving them?

These auction houses make it easier to balance things on the fly and offer interesting roleplaying aspects that are much more satisfying for my players then to walk into a shop - push cash on the table and leave with a magical weapon that lost all of its charms due to its ready availability or perceive the looted +2 axe in a dungeon as just another mundane-magic-item that you can buy in any major city.