Short Sale Transparency Act of 2013 - Directs the Director of the Federal Housing Finance Agency to require the Federal National Mortgage Association (Fannie Mae) and the Federal Home Loan Mortgage Corporation (Freddie Mac), in any case in which one or the other denies the purchase of a residential mortgage loan made to finance the purchase, under a short sale, of a covered dwelling on the basis that the purchase price involved is insufficient, to disclose promptly to the prospective seller the amount by which the purchase price was less than the minimum purchase price sufficient for approval of the purchase of such mortgage.