First, I'll need a letter of engagement including a detailed description of the scope of work.
Ok, back on track. You raised a very good point, that buyers' random unpredictability could somewhat reduce the Principle of Substitution's degree of certainty. My reply was that appraisals always consider a hypothetical typical buyer who, by definition, behaves predictably.
For the reasons I've already stated, I agree with you that there's no merit in attempting to forecast what a listing will eventually sell for. And (in my market anyway), there's absolutely no data indicating that asking prices will usually be discounted by a typical amount, or that there's any other way, short of appraising the listing, to make an educated guess. So, we both agree that a listing's asking price shouldn't be adjusted, but it doesn't look like you give the Principal of Substitution much validity. How come? I ask, because several others share the same view.