So, players enjoy having stuff. More than that, they tend to pick up stuff just to sell it - so why not have somewhere good to do so, rather than abstracting it away ("It's worth 20 gold." "Okay, you sell it - there's your gold!")
The Laughing Paladin (see previous post) will often sell basic supplies, like jerked meats, adventuring tools, etc. and will buy same when the group is really hard up for cash. You might be able to buy the classic "Things Off A Bloke Down The Pub", like magic beans (that may or may not be magic, and may or may not contain beans), mild narcotics (of the "Legal High sort, I assure you), perhaps even that woodcut of Elhonna, Goddess of Love, with a particularly, uh, friendly unicorn...
For more... exotic goods and services, Saanar's Miscellany is your best bet - run by Saanar, a mysterious Smoke Mephit, the Miscellany is a curio shop dealing in the odder planar stuff that people find on their travels. Find yourself needing an Aasimar's feather as a spell component? Saanar either has one, or knows someone willing (or not so willing) to donate one for the right price. And it's not just gold he trades in - he's very open to barter, and finding just the right weird thing can net you anything, so long as he needs it (or just really like the look of it). He'll even haggle most enthusiastically (creatures from the Quasielemental Plane of Smoke are happiest in urban environments - and what's more urban than a good haggle?), unlike many other specialist merchants of the Lower Ward.
The building itself is on a main-ish road (after all, Saanar likes to see as many people as possible). He can often be found outside, chatting to passersby, enjoying the dank and smoky air, or enjoying perfumed shisha in his back room.
Some sample items, for sale at reasonable prices:
Three teeth from a Slaad - specially treated, with unguents made from the raw stuff of Limbo, swallowing one of these teeth gives you the taint of a Chaotic being, for a short time (in game terms, add Chaotic as a bonus Aspect/3-dice Cliche for, well, as long as you want - one "scene" is advised. Good for fighting Lawful creatures, gaining access to specific magics, fulfilling ritual requirements, etc).
Feathers from an Avorinal - plucked from the wings of a powerful Celestial, these feathers can grant some protection against Evil creatures when burned and the smoke is inhaled (similar to the Slaad teeth, except it grants Good as your bonus Cliche/Aspect).
A lump of solid Smoke - taken from the Plane of Smoke, and specially treated, when cracked the item produces a harsh, thick cloud - great for making a stealthy getaway or causing general confusion and chaos.
A small knotted piece of string, soaked in water from the memory-stealing River Styx, and sealed with wax - while thinking about a piece of information, tie the string in a knot. You will instantly forget it, until you untie the string - you will get the memory back, as clear as if you'd just seen it. Can be passable to others (so they untie it and see your memory). Popular amongst double-agents, those looking to fool the Harmonium, and Sensates (other people's memories can be brilliant).