I'm a dumdum.

Party Like a Boxcar

https://backpack.tf/item/3628295924

Sale 1) Sold for a Duped Circling Heart Noble Amassment of Hats (27 Keys) + 5 Keys

https://backpack.tf/profiles/76561198053252981#!/compare/1536796800/1536883200

https://backpack.tf/profiles/76561198201196165#!/compare/1536278400/1537315200

The Noble was bought with most of the Keys you see in the second link.

27 + 5 = 32 Keys

Sale 2) Sold for a Neutron Star Human Cannonball (27 Keys) + 2 Keys

https://backpack.tf/profiles/76561198382654640#!/compare/1539993600/1540080000

28 + 2 = 30 Keys

Cannonball Mini

https://backpack.tf/profiles/76561197980218669#!/compare/1541635200/1541894400 - Sold for 30 Keys

https://backpack.tf/profiles/76561198382654640#!/compare/1540425600/1540512000 - Sold for 24 Keys + ~2 Keys of Items

https://backpack.tf/profiles/76561198070705963#!/compare/1541462400/1541548800 - Sold for a Scorching Flames Janissary Ketche

https://backpack.tf/profiles/76561198401509004#!/compare/1542240000/1542326400 - Sold for an Insomnia Cannonball + ~3 Keys of Items + 2 Keys

Ketche Micro

https://imgur.com/a/6TdWBxM - 3 Day Seller at 30 Keys, 3 Month Unsold at 34 Keys

https://marketplace.tf/items/30546;5;u61 - Sold for $33.33 (low) and $69 when Keys were ~$1.74 each

69 / 1.74 = 39.65, which rounds to 40 Keys

Calling this a high outlier.