https://forums.backpack.tf/topic/6033-rules-for-unusual-price-suggestions/

Use the value of keys on the site at which the item sold to determine the key conversion at the time of the trade. (i.e. use the SCM value of keys on SCM sales, use the marketplace.tf value of keys on marketplace.tf sales, use the opskins value of keys on opskins sales). For most transactions, this means you can use the key value at the exact date of the sale. When key prices are not stable, or when key values are unusually high or low at a certain day, a key average over a longer amount of time can be taken instead; a more detailed explanation can be found here.