Well, so as not to knock the bottom out the PC market, it'd be best if they sold stuff that PCs got through a lot of but didn't usually make. Alchemical ingredients have been good for this, and oil always sells. I think a possible solution would be healing potions at high but not ridiculous prices (say 60 for CSW) or affordable and useful ammo. Healing tends to be an issue as unless there's a mercenary-minded priest it tends to be people spamming Quest X that have far more than anyone else. Ammo is definitely a problem as it's a total crapshoot whether you can find any, since the only ways I've seen to craft bolts or arrows are a PrC or non-obvious powerful alchemy.
Scatter about stuff that everyone uses but mark it up enough that PCs are favourable as opposed to required, and we might have a proper NPC economy.
Scatter about stuff that everyone uses but mark it up enough that PCs are favourable as opposed to required, and we might have a proper NPC economy.