Sale at 8 keys (8th May 2020): https://backpack.tf/profiles/76561198341171690#!/compare/1588809600/1588896000

Seller's listing snapshot: https://cdn.discordapp.com/attachments/716014614298755085/730748150138011658/sale_1.PNG

Two sales on marketplace.tf on April 25 and may 15 2020: https://marketplace.tf/items/tf2/31024;5;u33

The classifieds on April 24 show a seller at 10 keys which disappeared a day after on the 25th when one sold on marketplace.tf for $20 https://bot.tf/stats/listings/4ZPO287X

The classifieds on May 13th show an other seller at 10 keys which disappeared by the 17th, sale on marketplace.tf at $19.99 https://bot.tf/stats/listings/4KVPR3L1

I'll also mention that there has been a sale at 6 keys 1 ref (6.02 keys), however it can't be used in the price because buyers are offering 6.11 keys

https://backpack.tf/profiles/76561198341171690#!/compare/1587945600/1588032000

https://backpack.tf/profiles/76561198158482500#!/compare/1587945600/1588032000