It strikes me that short of investigating every single sale, we have to make some assumption about whether all these sales are arm's length or not, if they are short sales or not, if they are otherwise, unique or impacted by external or functional issues or not. I mean any of the above can distort the data base. While a single such sale would not negate the statistics of a 100 sale sample, as
@The Bob noted above, there are times where market conditions are distorted. I recall what happened to Bartlesville, OK when Phillips 66 sold out to Conoco and was moved to Houston. A one-horse town collapsed. Suddenly housing, relatively cheap in the first place, was unsalable at any price and most of the non-top dogs were without a job or any prospect of one short commuting to Tulsa. That collapse was as close to overnight as such a collapse can be. It hurt a lot of people, many with flawless credit histories.
To me, anything short of 90 days it a hairshirt job trying to estimate whether a market is moving up or down. Market activity could be a hint, but in a slow market like today, we risk basing a price change upon insufficient data.
As an aside - Bartlesville was hurt by one Boots Adams - who was prez after the Phillips family passed the torch. Bartlesville also had the Cities Service (Citgo) company, but Boots was determined to force total loyalty upon the Phillips employees. If an employee had relatives who worked for Cities, they were fired. If they were caught buying gasoline from any other dealer than a Phillips station, they were fired, on and on. Boots was a tyrant, and Cities finally packed it in and moved to Tulsa. The town thus became vulnerable to a catastrophe due to the loss of the dominant industry and the subsequent satellite businesses who depended upon Phillips 66.