It's like physics in that way. "Assume a rod of length one meter with zero thickness and mass of one gram." Well, it can't exist, but we can do the math on it anyway.
In the same way, and idealized marketplace is one where the only forces are supply and demand. They meet at some point to determine price, the only output. Second order effects include the change in supply due to people buying and selling. Production and consumption can be added in, as can depreciation and advertising effectiveness. But these are all only the drivers of supply and demand.
In reality, players try to influence one or more of these inputs. Copyright and patent legislation can create artificial scarcity. Price caps and minimums can put floors and ceilings on prices. Contractual obligations can affect price, but can also disguise or manipulate the apparent supply and/or demand -- see Microsoft's "trade secret" contracts with OEMs. "Dumping" can artificially inflate supply as a long-term strategy to drive out smaller players.
Anti-trust legislation is intended -- in the view of "pure" marketplace economics -- to restore the "proper" functioning of a market that has been excessively manipulated by a player that can wield too many of the previously mentioned tactics.