I have not seen the article in question, but anyone who believes sales that are not in an MLS can not be valid comparable sales has not been around much. The fact is, there is no standard or regulation anywhere that suggests, much less supports, the claim. I spent 25 years working in markets where there were no MLS systems at all, many without any active realtors. The process was to visit the local courthouse, look at all deeds recorded within whatever period I needed to (often to the date of my previous visit), view and make noted from or copy the Assessor cards for those I printed off, take photos of those transfers, and go home to start calling buyers, sellers, brokers, auctioneers, and whoever would answer the phone. Maybe once or twice a year I relied on sales in an MLS to complete an SFR report. You use what you can get. Some say you can't use sales that sold with a contract for deed, or by auction, or were distressed, and on and on and on. Folks making those claims simply haven't yet learned to appraise.