Market makers do not control the supply of goods. They provide resting liquidity for pre-existing goods.

Similarly, market makers do not get to establish rules of the of their own "reality". Market makers are participants in a venue. It is the venue/exchange that sets the rules.

User Bengalilol seems to have inferred that because Valve made the venue, he can refer to them as the "market maker". This is not correct. Words have meaning. The meaning of market-maker is well-established in the context of exchanges. Market maker is incorrect terminology for Valve’s role.