Investment bankers are usually hired to manage the sale of securities in the primary market.
Note individual investors must hire a broker who is a participant in the ECN to execute a trade on their behalf.
Also, DMMs have some advantages in trade execution in exchange for assuming their obligations. They replaced what formerly were called specialist firms at the NYSE.
See Example 3.3 and 3.4 to illustrate the mechanisms of short-selling and margin calls on short positions, respectively.