BUY! BUY! SELL! SELL!
It so happens that there is not any real physical market for Steam where traders in cool jackets yell offers for DOTA Compendium points or scribble notes about Pyro Trading Card futures while they chug Red Bulls… although plenty of money is changing hands. Steam does however provide some nifty tools that give you something of the same effect when you are dealing with items that have been classified as commodities. On Steam (as in real life), commodities are any item that is sufficiently common that large quantities of them are traded and purchasers are primarily motivated by price (since the items are identical).