Magic Item Economy

Finding magic items is a great incentive for player characters to go adventuring. It rewards them with new powers for the risks they are taking.

Gary Gygax strongly maintained in the 1e DMG, that magic items of any kind should not be for sale, that players had to earn them through adventure at the risk of their PCs lives. After all, D&D is an adventure game, not a market simulation. 5e continues this tradition:

Magic items are gleaned from the hoards of conquered monsters or discovered in long-lost vaults. (DMG p. 135)

If magic items would be for sale would depend how rare and unusual magic is. In a world where nearly every village has some hedge wizard or temple with some cleric able to cast spells, and larger towns or nobles have competent, higher level spell casters, magic is common enough that there should be some economy for it.

At least simple spell scrolls, or common items like healing potions would be available for sale. In areas where there is a higher concentration of people and spellcasters, like large cities, there likely even would be markets, and shops or auctions for magic items like for rare and expensive art in our world, and if not, you need to come up with rationales why this would not be so.

Magic Shops

In larger cities there should be shops that cater to spellcaster's needs, buying and selling components, alchemical substances, inks, spellbooks, laboratory equipment and the like. In case the shop is run by a resident mage, he might sell simple scrolls, magic potions or spell services for appropriate prices. For prices, see the GM Screen on Goods and Services. 

Occasionally, you may with the right connections, be able to buy a more powerful magic item, maybe for performing a special service for the owner.

Town Mages

PCs wizards can scribe simple scrolls with moderate effort, and so can NPC wizards. Even wizards of moderate stature can make money from creating and selling scolls. Likewise they can cast useful spells with no direct cost to them as a service and charge handsomely for it to finance their research (cost of components to be covered by the buyer).

NPC wizards ocassionally sell common, low level spell scolls (mostly to spells that the PCs already have, a great way to limit access to new spells while still maintaining a believable economy). Spells like Protection from Evil and Good, Magic Missile, Detect Magic, Shield, Mage Armor or spells that are utter crap for typical campaign play and won't do much for the PCs like Jump, False Life or Illusory Script. 

Scrolls

The larger the city and higher level the wizard, the more likely one can find higher level scolls or spell access (also detailed on Goods). Anything over third level will be rare, and only found in the largest metropolises or dedicated magical societies.

Allowing the PCs to copy a spell should cost just as much as a scroll -- both can get the spell into the PCs spellbook, and while you cannot cast it directly like the scroll copying does not run any risk of losing it without copying it successfully. 

There is really nothing that PC wizards like better than copying new spells from NPCs in town, and they gladly would let them copy back as that would not even cost them money. Every new town for the PCs, the first stop is the town mage to see if he has any spells to sell or copy. Why do other wizards not have the same compulsion ? Why are the town mages not eager to swap? Why are there no markets for spell exchange?

The real reason of course is that it would make it too easy for the players to get new spells. The in game reason can be that the town mage is wary of competition: 

Wizards compete with each other for prestige, for winning service business from wealthy patrons, and for successfully gutting dungeons for treasure and magic.

• If I as a town mage equip another wizard with Alarm, Identify or Comprehend Languages, they may open a competing business costing me my lucrative trade. 
• If I give them Remove Curse, or Stone to Flesh, I lose my leverage to extract whatever large amounts of money and magic items I demand from them for a service they cannot get anywhere else. (Adventurers likely picked bread-and-butter spells like Fireball and Counterspell, that are more useful in everyday adventuring.)

• If I enable other wizards with divination and evasive spells like Clairvoyance, Wizard Eye and Dimension Door, they may use them to plunder my tower.

• If I provide them with offensive spells like Fireball, they may use them to hurt me. 

• If I am also an adventurer, or even as a resident mage want to venture out myself, we are directly competing. Anything I hand to them may be used against me.

In one game, the town mage allowed the players to copy Floating Disk, and then after he learned about a huge treasure trove they had come upon, watched with dismay as they used it to carry off most of the weighty coins they otherwise would have had to leave behind for him.

So, if anything, the town mage could making a counter 

NPC town wizards services should be unfairly expensive. The players have no alternative, and most likely do not have a good use for much of their gold anyways. It would not be uncommon for the wizard asking for magic potions or consumable items in exchange for a simple casting of a spell, on top of the gp cost. 

 

Temples

Likewise, simple healing potions can be fabricated by temples, to be given to the believers that are able to make an appropriate donation. 

Wizard Guilds and Libraries

Why are there no colleges, and libraries, where you can research spells? With a higher level of Magic, there could be. Access may be limited to members in good standing, and cost significant fees, as compiling, protecting and maintaining these troves of knowledge is both costly and in most places a monopoly. 

Guilds regulate the distribution of knowledge to secure their might. They have librarys full of spells accessible only to their members (and that can be used as the source for the "free" spells the PCs get to pick when gaining levels), who pay dues and have to follow guild law. To make these spells be generally available would undermine the guild's power, so the first law of most guilds is that spells may not be shared with outsiders. Countries and cities where guilds are established regulate all trade in magic, and typically block it.