So, I started with the "selling magic items" rules in the DMG and tweaked it until it did what I wanted.
FINDING AND BUYING MAGIC ITEMS
Magic items might be available for purchase in large cities, from the wizard's guild, or in exotic planar locales. In such locations, Common magic items are usually available for purchase at the standard prices (50-100 gp, or half that for consumable items), although it may take a day to find exactly what you are looking for.
Finding Magic Items
Finding Uncommon and rarer magic items requires 2d4 days of research. At the end of this time, make an Intelligence (Investigation) check, with the listed modifier for item rarity. If you are looking for a general category of item (such as "magic sword") use the rarity of the most common item of that type.
Rarity : Intelligence (Investigation) Penalty
Uncommon : -0
Rare : -2
Very Rare : -5
Legendary : -10
Intelligence (Investigation) Result
9 or less: Your research turns up only vague rumors, various legends, and fruitless dead-ends. You may always re-try, taking another 2d4 days.
10-14: You learn of an item that's close, but not quite what you were looking for. If you search for that item (taking another 2d4 days of research), you get a +5 bonus to your Intelligence (Investigation) check.
15-19: Acquiring the item requires a side quest: traveling, searching, infiltrating, investigating, or diplomacy, plus maybe an encounter or two. Roll on the Item Side Quest table below. Legendary items may require an entire adventure or a string of multiple side quests.
20 or more: You find the item's owner and they may be willing to sell it.
d8: Item Side Quest
1: The item was lost in the wilderness or at sea.
2: The item was entombed or lost in a dungeon.
3: The item was stolen; you know only who had it last.
4: The item is broken, and repairing it requires a difficult-to-obtain part.
5: The item's owner may be willing to sell it, but they are in a distant land, other plane, or extremely shady part of town, exposing you to travel-based random encounters.
6: The item's owner wants a low price for it (10% of base price) because it is cursed, stolen, broken, or otherwise troublesome; you get the item first and then dealing with the consequences constitutes the side quest.
7: The item's owner is unwilling to part with it at any price, for sentimental, religious, political, or arcane reasons. If you want it you will have to take it or steal it.
8: The item's owner will not accept cash; they will only trade it for a particular favor or other item (roll again for the side quest they wish you to undertake).
Buying Magic Items
If you contact the item's owner, make a Charisma (Persuasion) check to determine a final sale price.
Rarity : Base Price
Uncommon : 101-500 gp
Rare : 501-5,000 gp
Very Rare : 5001-50,000 gp
Legendary : 50,001-500,000 gp
Charisma (Persuasion) Result
4 or less: The item's owner adamantly refuses to sell it and will not engage in further negotiations.
5-9: 200% of base price, and the seller never wants to see you again.
10-14: 150% of base price, and the seller still feels you got the better deal.
15-19: 100% of base price, and the seller thinks this was fair.
20-24: 75% of base price, and the seller enjoyed doing business with you.
25 or more: 50% of base price, and the seller thinks you've done them a huge favor taking the item off their hands.
FINDING AND BUYING MAGIC ITEMS
Magic items might be available for purchase in large cities, from the wizard's guild, or in exotic planar locales. In such locations, Common magic items are usually available for purchase at the standard prices (50-100 gp, or half that for consumable items), although it may take a day to find exactly what you are looking for.
Finding Magic Items
Finding Uncommon and rarer magic items requires 2d4 days of research. At the end of this time, make an Intelligence (Investigation) check, with the listed modifier for item rarity. If you are looking for a general category of item (such as "magic sword") use the rarity of the most common item of that type.
Rarity : Intelligence (Investigation) Penalty
Uncommon : -0
Rare : -2
Very Rare : -5
Legendary : -10
Intelligence (Investigation) Result
9 or less: Your research turns up only vague rumors, various legends, and fruitless dead-ends. You may always re-try, taking another 2d4 days.
10-14: You learn of an item that's close, but not quite what you were looking for. If you search for that item (taking another 2d4 days of research), you get a +5 bonus to your Intelligence (Investigation) check.
15-19: Acquiring the item requires a side quest: traveling, searching, infiltrating, investigating, or diplomacy, plus maybe an encounter or two. Roll on the Item Side Quest table below. Legendary items may require an entire adventure or a string of multiple side quests.
20 or more: You find the item's owner and they may be willing to sell it.
d8: Item Side Quest
1: The item was lost in the wilderness or at sea.
2: The item was entombed or lost in a dungeon.
3: The item was stolen; you know only who had it last.
4: The item is broken, and repairing it requires a difficult-to-obtain part.
5: The item's owner may be willing to sell it, but they are in a distant land, other plane, or extremely shady part of town, exposing you to travel-based random encounters.
6: The item's owner wants a low price for it (10% of base price) because it is cursed, stolen, broken, or otherwise troublesome; you get the item first and then dealing with the consequences constitutes the side quest.
7: The item's owner is unwilling to part with it at any price, for sentimental, religious, political, or arcane reasons. If you want it you will have to take it or steal it.
8: The item's owner will not accept cash; they will only trade it for a particular favor or other item (roll again for the side quest they wish you to undertake).
Buying Magic Items
If you contact the item's owner, make a Charisma (Persuasion) check to determine a final sale price.
Rarity : Base Price
Uncommon : 101-500 gp
Rare : 501-5,000 gp
Very Rare : 5001-50,000 gp
Legendary : 50,001-500,000 gp
Charisma (Persuasion) Result
4 or less: The item's owner adamantly refuses to sell it and will not engage in further negotiations.
5-9: 200% of base price, and the seller never wants to see you again.
10-14: 150% of base price, and the seller still feels you got the better deal.
15-19: 100% of base price, and the seller thinks this was fair.
20-24: 75% of base price, and the seller enjoyed doing business with you.
25 or more: 50% of base price, and the seller thinks you've done them a huge favor taking the item off their hands.
