Buying and Selling Magic Items

As magic items are special to say the least, starting at "uncommon", going through "rare" and "very rare" and then still having 2 more levels, you won't just find a shop that sells magic items. In any normal setting finding stationary shops for most products in general, is already a rare thing. Most trade happens through paddlers and market stalls, even for hard to lug around things like books. The market for people buying and selling the incredibly rare and pricey magic items is infinitely smaller.
Therefor anyone searching for new items, or wanting to get rid of those in his or her possession, will have to work through intermediaries, who in turn will look for other parties wanting to buy or sell, and will charge commission for this job. The only other way for procurement of magic items is through finding them in ancient ruins or getting them from powerful and wealthy individuals.
Going back to the intermediaries, you will only find them in larger cities (large possible market) and even then they will have their work cut out for them trying to combine buyer with seller. Therefor there are two possibilities for intermediaries. The ones that only deal in Uncommon and Rare items, and the ones that will take on any job. The latter are easier to use if you need the real powerful stuff, but they will charge higher commissions on every level of item.
Prizing many different items takes many different values, find them here for 3rd edition. On might also use this, but it is often necessary to multiply the prizes there by 2 or 3.
Greater intermediary prizes (Lesser intermediaries half the commission):

Item Base=Top	Base Cost +1,5 Base	Commission	Buying Total +1,5 Base	Selling Total +1,5 Base
Armor +1	1000	150	1150	850
Armor +2	6000	750	6750	5250
Armor +3	24000	2600	26600	21400
Weapon +1	2000	300	2300	1700
Weapon +2	15000	1700	16700	13300
Weapon +3	40000	4500	44500	35500
Level 1 Spell	50	5	55	45
Level 2 Spell	250	30	280	220
Level 3 Spell	500	75	575	425
Level 4 Spell	750	90	840	660
Level 5 Spell	1000	125	1125	875
Level 6 Spell	1500	175	1675	1325
Level 7 Spell	2000	250	2250	1750
Level 8 Spell	3000	375	3375	2625
Level 9 Spell	4000	500	4500	3500
Ability +1	3000	500	3500	2500
Ability +2	10000	1500	11500	8500
Ability +3	30000	5000	35000	25000
Combinations 2	Total x 1,5	x 0,1	+ 0,6	- 0,6
Combinations 3	x 2	x 0,2	+ 1,2	- 1,2
Combinations 4	x 3	x 0,3	+ 2,3	- 2,3
Combinations 5	x 5	x 0,5	+ 4,5	- 4,5