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 must use the Key value on the exact date of the sale.