In a d20 world, a store that caters to mages, wizards, and sorcerers will have a wide variety of strange items, but none matches the variety of the Compleat Magic Store, which attempts to stock every spell component a wizard could need.
Every magic store will stock spell component pouches, spellbooks, parchment, and rare inks worth hundreds or thousands of gold pieces. A magic store that caters to divine spellcasters as well as arcane ones might stock some other things, such as papers scribed with holy texts and more expensive incenses.
Some components may be difficult to carry portably. Blood should be alchemically preserved, and other items that can spoil should be dried or pickled. Live insects can be carried in wire or wicker cages. Colonies of phosphorescent moss should be packaged in dark stone jars with a pamphlet on their care, including daily watering and sunning.
In most stores, the most valuable items carried will be gemstones, including 100-gp pearls, 1,000-gp black sapphires, 1,000-gp sapphires, 1,000-gp diamonds, 1,000-gp jacinths, valuable onyx and black onyx gems, as well as many hundreds of gold pieces worth of powdered diamond, emerald, ruby, sapphire, opal, jade, amber, crushed pearls, silver, and gold. Every magic store serving even a small number of adventurers should stock barrels full of diamond dust and crates of enormous diamonds.
The compleat magic shop should have several shelves with porcelain jars of tallow, gum arabic, bitumen, soft clay, brackish water, and vinegar, as well as one-ounce vials of sweat, tears, alchemically preserved blood, alchemically preserved black dragon blood, mercury, pitch, water, aqua vitae, aqua fortis, molasses, sweet oil, honey, giant squid ink, potions of bull's strength, and whitewash. Remember, there are eight drams to the ounce!
In addition to the above, the compleat magic store's inventory will consist of at least the following...
Groceries, including: wine, pork rind, butter, fat, raw meat, lumps of alum, and tiny tarts.
Tailored items and equipment, including: spools of thread, spools of string, spools of knotted string, small silk squares, squares of red cloth, patches of cured leather, gauze, miniature cloaks, tiny bags, small bags, soft cloth gloves, sturdy leather gloves, snakeskin gloves, and small leather loops.
Metalwork, including: small iron rods, small silver rods, silver pins, one-pound silver bars, thin sheets of lead, thin sheets of iron, bar magnets, copper wire, silver wire, gold wire, tiny bells, small pieces of mirror, small silver mirrors, tiny silver whistles, tiny steel razors, miniature brass hearing trumpets, small metal speaking horns, and brass keys.
Glasswork and ceramics, including: small glass rods, small sheets of glass, glass beads, glass cones, glass eyes, crystal beads, crystal hemispheres, crystal marbles, porcelain spheres, and porcelain cones.
Animal products, including: wool, beeswax, honeycomb, white chicken wing feathers, hummingbird feathers, owl feathers, eagle feathers, bat fur, cat fur, bloodhound fur, bull hair, fox hair, horse hair, dried bull dung, dried eagle droppings, dried fox dung, dried owl dung, dried bat guano, powdered animal hoof, eyelashes, basilisk eyelashes, rakshasa eyelashes, ogre mage eyelashes, pickled raptor eyes, pickled hen hearts, chicken bones, chicken eggshells, snake eggshells, snake scales, dried adder stomachs, dried snake's tongues, dried tentacles from a giant octopus or squid, powdered herring scales, spider webs, chameleon skins, turtle shells, dried grasshopper legs, dried fireflies, empty cocoons, and rotten eggs.
Vegetable products, including: phosphorescent moss, forked and straight twigs, dried rose petals, short reeds, licorice root shavings, powdered corn extract, powdered rhubarb leaf, powdered dried carrot, powdered garlic, skunk cabbage leaves, wood chips, wood splinters, powdered peas, nut shells, sesame seeds, mushroom powder, and saffron.
Mineral powders, including: soot, salt, sand, loam, grave dirt, earth from a ghoul's lair, powdered iron, granite dust, powdered lime, powdered charcoal, various colored powders, powdered mica, mica chips, talc, powdered sulfur, and powdered chrysolite.
Stones, including: flints, lodestones, granite cubes, square stone chips, copper pieces, zinc, iron pyrite, brimstone, agates, clear quartz prisms, sunstones, and pebbles.
Miscellaneous items, including: wooden statues of archery targets, darts, crystal rods stuffed with phosphorescent moss, clothing worn by ghouls, shards of bone from undead creatures, small candles, tiny fans, sponges, small clay models of ziggurats, tindertwigs. sticks of incense, will o'wisp essence. live crickets, live spiders, live fireflies, and pieces of iron from an iron golem, war machine, or hero's armor.
Many of the more expensive items a magic shop might sell will not be stock, but commissioned items. A well-run magic store should be able to contract craftsmen able to construct masterwork rings, wands, weapons, armor, staves, and most of the following: an exceptionally well-crafted an expensive chest, worth 5,000 gp, and a small replica; a jade circlet worth 1,500 gp; a doll of the mage, worth 5 gp; four strips of ivory worth 50 gp each; an ivory plaque worth 50 gp; a statuette of the mage carved of elephant ivory and decorated with gems, worth 1,500 gp; a miniature portal carved from ivory, worth 5 gp; a small piece of polished marble, worth 5 gp; a tiny silver spoon, worth 5 gp; a mirror of highly polished silver worth 1,000 gp; two statues, black and white, of canines with small iron bars attached (worth 50 gp altogether); a tiny lens of ruby or sapphire set in a small golden loop, worth 1,500 gp; a miniature platinum sword with a copper grip and pommel, worth 250 gp; and small forked metal rods of various sizes and materials.