Those entities which are in need of capital (i.e., they are in money shortage). The sell side could be a credit card company looking for an investment bank to package up its receivables into asset-backed securities (ABS) for resale, a commercial bank trying to resell portions of the loans it has made, a town looking for investors to sell its municipal bonds in order to finance new public projects (e.g., hospitals, schools, roads, etc), or a company trying to raise capital in order to finance expansion. The sell side is notoriously keen to obtain as much capital from the buy side, on the most favorable terms possible.

The sell side would love to get free or very easy money- with interest-free financing, no fixed term for repayment, and no promises about increasing the value of the investment. It is the various institutions that collectively form the financial system (intermediaries) that- in exchange for a fee- bring together the buy side and the sell side and negotiate a middle ground: the terms on which the buy side is willing to invest some of its capital and the sell side is willing to agree to in order to obtain that capital.