Bukkit

The default configuration is easily customizable. To make an item unavailable, simply drop that row in the database.
To add an item, insert with the following collumns and appropriate values. This can also be done with /market add in game

name
value
number
minValue
maxValue
changeRate
data
spread

name

Replace this with what you want players to type to buy/sell/price check.

value

Change the ##.## to whatever initial price you desire.

number

Change the ## to the data value of the new item.

minValue

The minimum value for the item

maxValue

The maximum value for the item

changeRate

the amount to change the value by with each item bought/sold

data

the aditional data value for the item. Usually 0, but different for items like colored wool

spread

Essentially, it's the difference between buy and sell prices. You'll want this to be > 0 to prevent abuse.