Remember that real world economics is very different from what is known as general economic theory. In practice we view economics as a system that is always attempting to reach equilibrium between Price,Supply,and Demand. However as a species we are full of imperfections and being the driving force of this system it is very hard for it to reach parity.

You are also assuming everyone will have some gold and wish to use it to buy items and only use real money to buy additional gold to bridge any gaps. But since items worth more than 4.5$ will likely be worth a lot more gold than one can easily farm, the amount of current gold you own may be negligible compared to the actual cost of the item you're trying to buy, in which case you would prefer to just buy it on the RMAH for cheaper (and yes it'll be cheaper on RMAH than on GAH if it's worth more than 4.5$, as the buyer gets to keep a bigger portion of the profit and thus wiling to sell for a bit less).
The last statement isn't always true in regards to sellers willing to sell for less just because they are getting a better deal on fees. This is called the profit margin, arguably on of the single most important statistics of a company. This is always trying to be increased by lowering costs(fees) and increasing price of goods or services sold.

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