Mt. Gox didn't let you withdraw real currencies other than JPY, most interestingly the dollar, from approximately last August. This meant that when you were selling BTC for USD, you were actually accepting a USD liability of Gox -- Bitcoiners sometimes called them GoxBux or GoxUSD. It's a number in a database and now, apparently, something which you'll eventually be allowed to assert as a claim in their liquidation, but it is not actually a dollar.
The failure to allow withdraws in currencies they supported is why many people were speculating that Mt. Gox was insolvent, for much of the year prior to them admitting insolvency. As it turns out, we were probably right.
I don't follow your argument. The person selling the bit coin in 2 assumes that when the number in their account balance goes up by USD_FAKE that there was really money to back it up. They have no way of knowing that Mt Gox didn't ever have that money.
I think that is the point they are trying to make.
Assuming "Joe" represents the entire userbase of Mt. Gox, which deposited its own money into the system:
Start : (Markus: $0) (Joe: 1BTC, $200)
Step 1: (Markus: $1200) (Joe: 1BTC, $200)
Step 2: (Markus: 1BTC) (Joe: $200 + 1200)
Step 3: (Markus: $200) (Joe: 1BTC, $1200)
Assuming Markus withdraws fiat before Joe does, Mt. Gox only had $200 in real money and $1200 in fake make-believe numbers, so Joe is left unable to withdraw.
But it seems like if they were pumping the price up, then a lot of active traders would be holding fake USD. Mt. Gox can't tell if it is accepting real or fake USD while dumping. I think my first post outling a possible scenario that generate nothing except perceived loss to users once they can't withdraw.
When a user does not yet have sufficient USD on deposit with Gox to buy the bitcoins they want, they deposit more, and MtGox now has more real USD under their control. It's not a matter of selling the coins for real vs fake USD.
I wonder if the information even exists to create a ledger of all the real dollar deposits and withdrawals that Mt. Gox carried out. I suppose even if that ledger made sense they could be sending the $200 to an account (or multiple accounts) controlled outside of the business.
Step 2: buy BTC at $1200.
Step 3: sell BTC at $200.
Step 4: profit!