Money can also be created if traders achieve a negative account value (such accounts become inactive, possibly permanently).
Money can be removed from the market if traders lose money to the automatic market makers, or if traders have their accounts suspended (e.g. for creating multiple accounts used by a single trader, or for colluding with other traders in such a way as to make a trader's performance misleading).
Also, note that, because the automatic market makers will often lose money, the typical trader in this market will have a positive return on investment.