Sale 1:

Sold for Pro. KS Australium Medi Gun (42)

No outpost

Seller: https://backpack.tf/profiles/76561198148511293#!/compare/1517443200/1517529600

Buyer: https://backpack.tf/profiles/76561198038860439#!/compare/1517443200/1517529600

X = 42

Sale 2:

Sold for 31 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1520899200/1520985600

Buyer: https://backpack.tf/profiles/76561198036270946#!/compare/1520812800/1520985600

Confirmed by bot.tf stats: https://i.imgur.com/YXSwIpI.png

X = 31

Sale 3:

Sold for 29 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1522281600/1522368000

Buyer: https://backpack.tf/profiles/76561198017911489#!/compare/1522195200/1522368000

Confirmed, again, by bot.tf stats: https://i.imgur.com/YXSwIpI.png

X = 29

Sale 4:

Sold for $50 on Marketplace.tf

https://marketplace.tf/items/1115;5;u3001

https://backpack.tf/item/4199237264

50/1.98 = ~25.25252525252525

X = 25

Sale 5:

Sold for Puzzle Hellmet

Seller: https://backpack.tf/profiles/76561198034681010#!/compare/1518393600/1518480000

Buyer: https://backpack.tf/profiles/76561198354766938#!/compare/1518393600/1518480000

Hellmet has 2 sellers at 23 keys, which is lower than quickbuyer price on the rancho. Excluding.

Sale 6:

Sold for 20 keys + some ref

Seller: https://backpack.tf/profiles/76561198354766938#!/compare/1518912000/1518998400

Buyer: https://backpack.tf/profiles/76561197994729298#!/compare/1518912000/1518998400

Even lower than the hellmet sale, excluding.

Sale 7:

Bulk for Misty Skull Liquidator’s Lid

Seller: https://backpack.tf/profiles/76561198036270946#!/compare/1521158400/1521331200

Buyer: https://backpack.tf/profiles/76561198337504240#!/compare/1520985600/1521158400

Sellers:

~27.75 keys:

https://i.imgur.com/YE7yh08.png

https://backpack.tf/item/2708750534

28 keys:

https://i.imgur.com/JiXPtsL.png

https://backpack.tf/item/4255056243

29 keys:

https://i.imgur.com/NKRlEGW.png

https://backpack.tf/item/2718388331

80 keys:

https://i.imgur.com/96mJSL4.png

https://backpack.tf/item/2720659167

84 keys:

https://i.imgur.com/kORw2I1.png

https://backpack.tf/item/2690642332

90 keys:

https://i.imgur.com/zG84rUh.png

https://backpack.tf/item/3314858191

Sales at:

25

29

31

42

42 is an outlier, excluding.

Going with 25-31.

Here's the quickbuy listing at ~23.4 keys before it was deleted:

https://i.imgur.com/XaiwisL.png

Given that sales 5 & 6 went for lower than the quickbuyer at the time, it might not be a bad idea to keep it excluded, but i'll wait for someone else's opinion on that.