You are all forgetting the withdraw limits. MtGox and similar have 100BC a day (bitcoin7 has 50BC a day and I think one has 500BC a day). MtGox and most have $1000 a day with $10,000 max a month
Let's say you have 20,000 bitcoins that you want to sell right now and you don't care how low the price goes.
1) You make accounts with each trading site.
2) You make a dwolla account and confirm it to your bank account.
3) You transfer in enough coins to make it worth $1000 without going over, which at most prices is 180 coins.
4) Then you sell on each site.
5) Then you withdraw the coins.
6) Then you withdraw dwolla to your bank account.
Okay now you got more coins.
7) Change your IP address. This takes 2 minutes.
Repeat step 1. Do step 2 but with a fake name and phone # and a different bank account (keep your real first and last name). Do steps 3-6.
9) Repeat step 7 and 8 about ten more times to get 20,000 coins sold and transferred out. You might run out of bank accounts so you will either have to open new ones or wait till the next day and likely you will just end up waiting.
The result is it takes a long time so no one is going to sell 20,000 at once to cash out for use.
Why not skip all the trouble and just ask mtgox to increase your limits?