The're taking a 5% tax off each transaction, not feakin 5,000 Gil off each transaction. If the community is rich, then the tax is high, if too much Gil is taken off the economy, the community is poor and items sell at lower prices, thus lowering the actual tax too. Imagine an extreme situation every item is 20Gil, every transaction has a tax of 1 Gil. God you can play Monopoly all day without even noticing a Gil difference. Market's deflate and inflate on their own. The actual flunctuation can be dependant on the current influx of new players but it will still run in a cycle.

Just stop using the market as an excuse.