Thank you very much for the detailed response.
The sales of other units were in 2017, so there was no data to use in the prior year when it was converted. Allowing for a meager $5K of appreciation over a 9 year period doesn't seem unreasonable even with the market drop of 2008/2009 since the conversion was 7-8 years after the drop. Hindsight is 20/20, but we weren't aware of those prior sales until 2020 when the realtor listing the property pulled up prior sales to determine the listing price.
I don't know what documentation would be required to "prove" that 2016 valuation other than what I just stated was apparently a very conservative and reasonable estimate of value.
In hiring a CPA, how would he/she come up with the value of the furnishings and appliances to determine whether the original values used were too high? I don't know what sort of documentation would need to be provided, as none exists. The property was never anticipated to be turned into a rental when it was purchased so no receipts exist.
