It makes total fucking sense really. If a 90 key hat with no sales has a single seller at 60 (30% off) and it doesnt sell after 22 days, why on earth should the hat still be valued at 90? 60 is more accurate as thats what the only seller on the market even values the hat at. Its just dropping with a B/O greater than 15% off at at least 15 days unsold with no sales. Read the unusual guide

I think the idea is that sales are always better than sellers, so giving it the 15 days allows some time for actual sales to occur. Sales give a price point, whereas sellers really only give a ceiling, but no floor.

That, and maybe also that waiting 15 days sorts out whether or not the sellers are just quickselling below the item's market value. So in this instance, if one or more are listed and sell very quickly at ~55 when the price is 90, they might just be quicksales. However, if they sit there at almost 50% off the current price for 15 days and no one buys, that's a pretty big clue that the current price is too high. Then you can proceed and "those are just quicksales" is not a valid counter, because they didn't move at that price.