My Realms game:
I do a quick rundown of the town using population, GP limit and highest level NPCs (DMG tables 4-40, etc). I make some quick notes describing the higher level NPC spellcasters (attitude, agendas, etc). I usually do this for the nearest towns before the game so that the information is readily available at play.
PCs can use Gather Information to find mages amenable to doing comission work, then use social skills to negotiate out a price for an item. The NPCs class and level (and the result of the negotiation), determine what items he can craft and at what price. Generally mages are willing to trade for items they can personally use. Same goes for high-level clerics.
Red Wizard enclaves and local temples handle the small stuff (under 2000 gp, mostly potions and scrolls), selling them openly.
Good social rolling (Gather Info, Bluff) may get you a private meeting with the local black market: usually rogues, smugglers, or Red Wizards going 'under the table'. There some of the more esoteric and dangerous stuff can be bought and sold (generally items that are close to the town's GP limit but the local mages won't or can't deal with).
I try to keep it brief, brisk and balanced.