IDBers are traders' minions (jk).
Trading is quite a broad field. At i-banks, their job is split into two: agency trading and flow trading. Agency is in essence acting as a broker for large institutional clients.
Agency:
If Pepsi wanted to sell their 1m share stake within say, Apple. They'd call up the trader at the bank and him and and his team will break down the trade into block trades to be executed in such a way that it doesn't affect the market at large.
They'd price the securities, figure out when to enter and exit and how to place the order. In comes the IDBer, the trader would contact the IDBer to place their trade through them - i.e. the IDBer is executing the trade on behalf of the trader at the bank who is in turn making trading decisions on behalf of Pepsi.
The result is the trader at the bank makes commission from Pepsi, and the IDBer executing the trade makes commission from the trader.
Flow:
Say McDonalds wants to buy 2mm shares of Citi. The trader at the bank would make a market by simultaneously offering a bid and an ask price. So what they'd do is use the bank's inventory (i.e. the bank's money) to purchase these 2mm shares, then they would sell the 2mm shares onto McDonalds - making money from the difference between what they sell it for and where they bought it at (the spread).
In this case, the trader would engage with an IDBer to connect themselves to a seller of 2mm Citi shares whom they can buy from (IDBer takes commission for this transaction), and then trades these newly acquired shares with McDs directly.
IDBers are effectively executing trades for and connecting large-scale buyers and sellers (mostly banks and other financial institutions).
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