Think a little more about it. You have a trading platform. Someone gives you 1000 Euro, anotherone 10 BTC. No trade yet.
Now the exchange rate is rising from 100 Euro/BTC to 500 Euro/BTC. Now a trade happens, 2 BTC for 500 Euro each.
There is also 1000 Euro and 10 BTC on your trading platform, as before, nothing really changed for the trading platform (but for the fee).
Offers are possible only for 1000 Euro. Your failure is the 2500 Euro, which does not exist for changing.
Correct me if I'm wrong but does that not just make Mt Gox a Ponzi scheme? Because they have to keep relying on new customers to buy Bitcoins to pay customers who are cashing out?
...
No, has nothing to do with a Ponzi scheme. And yes, they relay on new customers.
Think about having a trading platform, and one customer gives you 10 BTC. He now can offer this BTC for any price, 500 Euro/BTC for example.
This will not matter as long as you do not find another customer, which gives you Euros. Without Euros no trade.
Two customers have an agreement about the price of one bitcoin, and if they agree they swap the currencies. This has no influence on the assets of the exchange platform, they earn the fees only. And they have to keep the deposits of the customers save and separated from their own assets.
