What term refers to the buying and selling of existing mortgages, often as a part of a 'pool' of mortgages?

The term that accurately refers to the buying and selling of existing mortgages, often as part of a 'pool' of mortgages, is commonly understood as "Mortgage-backed securities." These securities are financial instruments backed by a bundle of mortgages that are sold to investors. When mortgages are pooled together, they provide a way for investors to gain exposure to the cash flows generated by the underlying mortgage payments.

Mortgage trading typically refers to the process of buying and selling individual mortgages rather than dealing with pooled assets. Second markets are usually referred to as secondary mortgage markets, where existing mortgages and mortgage securities are traded, but they are not defined as a term in the question. Mortgage syndication involves multiple lenders coming together to fund a single large loan, which is different from the trading aspect of existing mortgages.

Therefore, the correct term for the buying and selling of existing mortgages, particularly in the context of pooled mortgages, is "Mortgage-backed securities." This industry practice plays a crucial role in the mortgage market by providing liquidity and making funds available for new mortgage loans.