History: https://backpack.tf/item/9578950078

Sold for 19 keys:

https://backpack.tf/profiles/76561198220098345#!/compare/1616803200/1616889600

https://backpack.tf/profiles/76561198198440035#!/compare/1616803200/1616889600

Added 13 keys for a Strange Aussie Scattergun:

https://backpack.tf/profiles/76561198198440035#!/compare/1616803200/1616889600

https://backpack.tf/profiles/76561198799601783#!/compare/1616803200/1616889600

The Scatter's current price was about right at the time of the sale (37 keys) https://gladiator.tf/sales?item=Strange%20Australium%20Scattergun&at=2021-04-27T17:50:21.589Z https://gladiator.tf/listings/605fd175ada0869639328d4a

37-13=24 keys, no buyout

Untraceable:

https://backpack.tf/profiles/76561198799601783#!/compare/1616889600/1616976000

https://backpack.tf/profiles/76561198311400236#!/compare/1616889600/1616976000

Sold for a Phosphorous Tsarboosh + 5 keys:

https://backpack.tf/profiles/76561198311400236#!/compare/1616889600/1616976000

https://backpack.tf/profiles/76561198843683108#!/compare/1616544000/1616976000

Tsarboosh is in-date at 26-29 keys (mean of 27.5 ~28 keys) https://backpack.tf/stats/Unusual/Tsarboosh/Tradable/Craftable/63

28+5=33 keys, buyout was 37, doesn't cap https://gladiator.tf/listings/606239460541b4c0fb991089

Added 30 keys for a Laugh-O-Lantern Pithy Professional (bulk):

https://backpack.tf/profiles/76561198843683108#!/compare/1619049600/1619136000

https://backpack.tf/profiles/76561198066238567#!/compare/1619049600/1619136000

Sold for 21 keys:

https://backpack.tf/profiles/76561198066238567#!/compare/1619136000/1619222400

https://backpack.tf/profiles/76561198360428856#!/compare/1618876800/1619136000

Sales at 19, 21, 24, 33

The Pithy Professional which was involved here didn't have a buyout itself but there was one for sale at 35 keys at the time of the sale https://gladiator.tf/listings/6080b559497dd09c4f20355b

This means the Cap was most likely valued at less than 33 keys in that sale. Given this and the fact 33 is far away from the other sales, I think it's enough to call that sale an outlier. There's also a seller at 32 who has been there for 4 days https://backpack.tf/item/10100849073 (not really long enough for counter evidence but it supports calling 33 high)

Gonna stick with 19-24 for now. Keeping 19 included because it took some time to sell