There is an issue with this suggestion, the War Paint has an unsold at 13 Keys: https://backpack.tf/classifieds?item=War%20Paint&quality=11&texture_name=Saccharine%20Striped&wear_tier=2&killstreak_tier=0

The premise for this suggestion falls apart if the sale would've been an outlier in the War Paint itself. This methodology makes sense when the premise is that the sale in question matches or is lower than the war paint, but when it's higher it doesn't really make sense to value the weapon above the war paint, when a war paint can be bought for less and applied to the same weapon for a price lower than the suggested price.

For this reason, I believe this should stay unpriced for the time being unless there's more sales you haven't been able to find.