MBS Investor MBS InvestorFor MBS Mortgage Loans, either a Third Party MBS Investor for Lender-Arranged Sales, or the Multifamily Trading Desk. for an MBS Mortgage Loan MBS Mortgage LoanMortgage Loan purchased by Fannie Mae in exchange for an issued MBS backed by the Mortgage Loan. that is not the Multifamily Trading Desk Multifamily Trading DeskTeam that: purchases and trades Cash Mortgage Loans and MBS Mortgage Loans for Fannie Mae's own account; and can be contacted at (888) 889-1118. .