One of the things that is a bit of a paradigm shift from previous editions that keeps coming up around magic items is that magic items are not something you can buy/sell. I assume this is really only in regard to permanent items. But this is strange to me. Assuming a typical D&D campaign setting where magic is a reasonably common thing and while it might be rare for an individual to be able to produce permanent magic items, certainly the history of the setting is long, magic items are very valuable and rarely destroyed and the people who make magic items that are useful for fighting horrible monsters are probably rarely the same people who use them for such purposes (or at least that is my operating assumption). So while I agree there isn't some vast magic industrial workshops producing +1 longswords for the local magic box store, what is the mechanism by which items get distributed from magic item producers to magic item users? If not a market (or some other mechanism that is reasonably modeled in game by the exchange of treasure for items) then what is it and what is the use for mundane treasure to an adventurer if not to upgrade gear? What do PCs do with items they find not very useful?
In my own campaigns I've always been upfront with the "rules" for finances and treasure with a few guidelines I provide to my players:
1) Adventurers do not spend all of their time actually adventuring. A typical "adventure" or set of related adventures might cover 2-3 levels and then there is down time that could be a matter of a few weeks at low level to multiple years at high levels (the assumption is there are more opportunities to save the village than to save the world/universe/multiverse)
2) During this downtime the adventurers earn some sort of living with an income that roughly matches their lifestyle.
3) This leaves the treasure they have acquired from adventuring to be used for such things as buying better gear, commissioning the creating of a special magic item, influencing the world in various ways, etc.
While I don't have magic shops that are the equivalent of our world's sporting good stores for the purchase of any and all magic items listed in the various source books, I am generally fairly liberal about allowing PCs to buy/sell/trade magic items during this time. The assumption is that PCs didn't go to a local store...they did some research, made some contacts, bartered, whatever. I just don't want to waste table time for a PC to sell his +2 battle axe that isn't all that useful to him for a low price and buy a +2 longsword with the proceeds (plus a sizable chunk of gold) for a higher price if that is what the player wants for his character.
I like this method because as a DM I want magic items I place in the game to either have a specific reason for being there or make sense (in the game world). I also want non-magical treasure to seem like an actual reward that the players find useful and I don't really want them to have to use it in ways outside of adventuring and/or influencing the game world. I don't think offering a reward of 1000gp for completing some quest means much of the players don't feel like they can do something cool with it and the most cool thing they can do is get that magic item they've been saving up for.
Anyway...that's just me. I don't really need rules and guidelines or rules for this but I imagine newer players find them useful.
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