You still don't understand the difference. That's fine. I'll explain to you why your example doesn't mean what you think it means.
A house in the county appraised by a licensed real estate appraiser for $2.8M. That's the FMV, based on the accepted method of calculating FMV. Once on the market, someone overpays for the property. That doesn't change the FMV, it just means that someone over paid. Someone could just as easily pay less than the FMV, it still wouldn't change the FMV.
You seem to believe that FMV is some number that people pull out of a hat. It's not. It's a methodology that real estate appraisers use and is accepted. There are software programs that will do the calculation for you. What someone subsequently pays doesn't change the FMV for the time that it was calculated.
I've argued FMV calculations in front of property tax boards. Those FMV numbers are produced by licensed professionals. Even if you bring in dueling experts, your appraiser has to be applying a known appraisal methodology. You can't roll into court and say "I think the property is worth $XX." The very first question that has to be answered is "Where is your appraisal report, prepared by a licensed real estate appraiser".
Hence Trump's problem. There's a licensed appraisal report out there, prepared by an expert and used by the county, that sets the value at $28 million. That's the number. If you can get a sucker to pay you more or you're desperate and sell it for less...it doesn't fucking matter. The FMV is still the FMV.