Including 5 because it's a common trade point.

https://backpack.tf/item/4412678958

Sale 1) Sold for 7 Keys + 1 Refined

https://backpack.tf/profiles/76561198021055079#!/compare/1543276800/1543363200

Rounds to 7 Keys

https://backpack.tf/item/4412762908

Sale 2) Bulk sale

https://backpack.tf/profiles/76561198056176130#!/compare/1544486400/1544572800

https://backpack.tf/item/4439787587

Sale 3) Sold for a Roaring Rockets Scotman's Stagger

https://backpack.tf/profiles/76561198142424997#!/compare/1541203200/1544054400

Taunt for Taunt sales aren't the best to use.

https://backpack.tf/item/4621889324

Sale 4) Sold for 6 Keys

https://backpack.tf/profiles/76561198832624621#!/compare/1546041600/1546128000 - Seller's Backpack

https://backpack.tf/profiles/76561198804931698#!/compare/1545868800/1546128000 - Buyer's Backpack

The Stickybomb sold for the 6 Keys that were used to buy the Pipes.

https://backpack.tf/item/4929976311

Sale 5) ?

https://backpack.tf/profiles/76561198815542934#!/compare/1542326400/1542412800

https://backpack.tf/profiles/76561198029371596#!/compare/1542326400/1542412800

https://backpack.tf/item/5386657347

Sale 6) ?

https://backpack.tf/profiles/76561198042745046#!/compare/1543622400/1543968000

Sale 7) Sold for 6 Keys

https://backpack.tf/profiles/76561198832624621#!/compare/1544313600/1544572800

https://backpack.tf/item/5635877678

Sale 8) Sold to a Buy Order for 5 Keys + 11 Refined

https://backpack.tf/profiles/76561198092349526#!/compare/1542326400/1542758400

Rounds to 5 Keys

Sale 9) Sold for 7 Keys

https://backpack.tf/profiles/76561198021055079#!/compare/1542672000/1542758400

IDs match.

https://backpack.tf/item/5738818169

Sale 10) Sold for 6 Keys

https://backpack.tf/profiles/76561198132182386#!/compare/1542499200/1542585600

https://backpack.tf/item/6754271545

Sale 11) Sold for a Roaring Rocket Zoomin' Broom

https://backpack.tf/profiles/76561198261509772#!/compare/1541030400/1542758400

https://backpack.tf/profiles/76561198115381172#!/compare/1541116800/1541203200

Taunt for Taunt sales aren't the best to use.

https://backpack.tf/item/4455632284

Sale 12) ?

https://backpack.tf/profiles/76561198412458800#!/compare/1541289600/1541376000

https://backpack.tf/profiles/76561198170835742#!/compare/1541289600/1541462400

Sale 13) Sold to a buy order for 5 Keys + 9 Refined

https://backpack.tf/profiles/76561198170835742#!/compare/1541721600/1541894400

Rounds to 5 Keys

Sale 14) Sold for 5 Keys?

https://backpack.tf/profiles/76561198158206236#!/compare/1543017600/1543104000

Sale 15) ?

https://backpack.tf/profiles/76561198089253101#!/compare/1543276800/1543363200

https://backpack.tf/profiles/76561198073381086#!/compare/1543363200/1543449600

Sale 16) ?

https://backpack.tf/profiles/76561198073381086#!/compare/1543536000/1543622400

https://backpack.tf/profiles/76561198041524603#!/compare/1542931200/1543622400

Sale 17) Sold to a buy order for 5 Keys

https://backpack.tf/profiles/76561198041524603#!/compare/1543708800/1544486400

https://marketplace.tf/items/30671;5;u3021 - Sold for $6.12 (low outlier) and $10 when Keys were ~$1.80 each

https://marketplace.tf/items/5021;6/Mann%20Co.%20Supply%20Crate%20Key

10 / 1.8 = 5.56, which rounds to 5.5 Keys