Buy-side vs. Sell-side

A look at what this common Wall Street terminology means:

TheStreet.com says:
" Sell-side firms are what people traditionally think of when looking for Wall Street jobs. These firms underwrite securities and advise on mergers and acquisitions through their corporate finance divisions. Their sales and trading divisions make secondary markets in a variety of securities, including stocks, bonds, currencies, swaps, commodities and derivatives. Analysts in research divisions make both macro (overall investment strategy) and micro (company-specific) recommendations.

Buy-side firms generally manage portfolios on behalf of clients. They include insurance companies like Aetna, investment management firms like Wellington Management, mutual fund companies like Fidelity and hedge funds like Moore Capital."

A clearer definition (I think) is this: the key is to think of basic financial instruments (stocks, bonds and the like). Buy-side firms (such as mutual funds) invest in these financial instruments. Sell-side firms sell these - for example, investment banks may underwrite a company's IPO, or help to sell the company's shares to the public (and take a cut).

Though the definition seems clear-cut, its not always this simple.
Where do commercial banks stand (they take deposits and issue loans)? They invest the deposits by buying securities - so buy-side? Or maybe - because they issue bank loans, a form of financing- sell-side? Officially, they're on the buy side.
Insurance companies sell what might be called a financial instrument, but they fall on the buy-side because they invest the policies they collect in securities. Investment bankers who broker M&A deals are on the sell-side, regardless of whether they represent the company that's being taken over (sell-side?) or the raider company (the one that's looking to 'buy').