The case where they are selling you something and are SHORT the product is a special case which requires more work usually. The most egregious case was the ABACUS deal (http://www.theworkingeconomy.com/simple-explanations-of-econ...) where Goldman was actively short the product and needed a dumb customer to unload the other side (enter Fabulous Fab and his widows orphans: http://www.reuters.com/article/us-goldman-emails-idUSTRE63O2...)
The case where they are selling you something and are SHORT the product is a special case which requires more work usually. The most egregious case was the ABACUS deal (http://www.theworkingeconomy.com/simple-explanations-of-econ...) where Goldman was actively short the product and needed a dumb customer to unload the other side (enter Fabulous Fab and his widows orphans: http://www.reuters.com/article/us-goldman-emails-idUSTRE63O2...)