You can use a CMA (by a real estate agent or broker) for FMV, but the IRS considers the best evidence of FMV to be an appraisal by a certified real estate appraiser.
The sale within "six months" as acceptance of FMV on the date of death appears nowhere in the Code or Regulations and those propounding that notion may be conflating an estate tax valuation rule (alternate date) with the general rule.
