For example, an AIG salesman sells a Credit Default Swap to Goldman Sachs. AIG is valuing that CDS assuming the underlying instrument will not default. Goldman Sachs buys it because they have valued it more correctly (their model assumes that there is a risk of default??).
Bottom line, if you sell something at too low a price, customers are going to pull a truck to your door to buy it.
If I was a salesman selling Gold, and I was able to sell at $600/oz, I would have no problem selling a massive amount, and making a great commission. (Note: I'm not responsible for procuring the gold, my role is just to sell it, and make a commission on the amount sold).