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).
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.