Most versions of D&D provide XP values for monsters, usually awarded for defeating the monsters in combat. The Third Edition (and Pathfinder) manuals also give explicit prices in gold pieces for every enchanted item. Here is an idea for modifying how advancement is handled using those numbers. XP value specified per monster is converted into GP. This is the value of the monster’s treasure (and equipment). XP is given for GP spent, as I have been doing in my Vaults of Pahvelorn game. Additionally, magic items can be purchased as is commonly expected (I think) in many Third Edition games.
In a game like this, I still wouldn’t make magic item shops common. Instead, I would put together some sort of system to rate sellers of enchanted items (cunning folk, wizards, specialist merchants, demonic patrons, and so forth). The rating could either be a flat GP threshold (this NPC can sell magic items of up to 1000 GP value) or it could be something like an inventory saving throw (this wizard has a stock number of 11 — roll 1d20 against that target number, modified by desired magic item level). This is somewhat similar to an idea I had for rating sages, which I should probably also write a post about. This creates a motivation to quest for NPCs that are better able to provide powerful items. Particular famous items of power might be the purview of specific individuals.
The entire advancement system could even be replaced by item acquisition, for a slightly lower power game. So, for example, fighters wouldn’t ever get attack bonuses, they would just get more powerful weapons. Wizards wouldn’t prepare spells, they would buy (or create) scrolls and other enchanted devices.
The interesting aspect of this proposal is that it maintains most of the expectations of the 3E/Pathfinder advancement system (assuming that standard class progression is also maintained) while only modifying the specific action incentive. The standard benefits of XP for GP are preserved (rewarding clever planning as opposed to straightforward combat). Obviously, this system would result in a setting with more common magic, but that’s not necessarily a bad thing.