First, I set a level limit based on the city. For example, a place like Sharn might have a variety of level 1-8 magic items.
Then I reserve the right to say 'it's not available' for certain items whose properties or powers are a little too extraordinary. Grasping longspears might be around, but transference hammers are not.
And then in cases I'm uncertain I assign a probability of availability and then roll a dice based on that probability. (i.e. 50% chance = call three numbers on a d6 and sell the item if they come up)
Then I reserve the right to say 'it's not available' for certain items whose properties or powers are a little too extraordinary. Grasping longspears might be around, but transference hammers are not.
And then in cases I'm uncertain I assign a probability of availability and then roll a dice based on that probability. (i.e. 50% chance = call three numbers on a d6 and sell the item if they come up)