Sale 1:

Sold with ~11 keys in items for Death at Dusk Tsarboosh (37)

http://www.tf2outpost.com/trade/29235812 - B/O of 30

Seller: https://backpack.tf/profiles/76561198211000936#!/compare/1499472000/1499558400

Buyer: https://backpack.tf/profiles/76561198038296415#!/compare/1493596800/1499817600

X = 26

Sale 2:

Sold for 25 keys + Uncraftable High Five (<0.5)

Seller: https://backpack.tf/profiles/76561198056472266#!/compare/1495670400/1495929600

Buyer: https://backpack.tf/profiles/76561198202299969#!/compare/1495843200/1495929600

X = 25

Sale 3:

Sold for 23 keys + 14 ref (<0.5) in ~2 days

Seller: https://backpack.tf/profiles/76561198118507462#!/compare/1493942400/1494028800

Buyer: https://backpack.tf/profiles/76561198056472266#!/compare/1493769600/1494028800

X = 23

Sale 4: – Credit to Abhijeet

Sold for 20 keys

Seller: https://backpack.tf/profiles/76561198293749329#!/compare/1497744000/1497830400

Buyer: https://backpack.tf/profiles/76561198211000936#!/compare/1497744000/1497830400

http://i.imgur.com/XBfd7sr.png

X = 20

Sale 5:

Sold with 12 keys for Eldritch Flame Hellmet

Seller: https://backpack.tf/profiles/76561198088583151#!/compare/1496880000/1497312000

Buyer: https://backpack.tf/profiles/76561198293749329#!/compare/1497225600/1497312000

Should be used on the Hellmet.

Sellers:

35 keys:

http://www.tf2outpost.com/trade/28647433 - 7 months, probably lowered at some point

45 keys:

http://i.imgur.com/ZziBf6R.png - 3 months

http://i.imgur.com/o40LkBE.png - 3 months

Buyer at 20:

http://i.imgur.com/dJ4itba.png

Sales at:

20

23

25

26

Buyer at 20. Excluding 20.

Given that, according to the seller of sale 4, it struggled to sell at 20, I will not exclude 23. Couldn’t find the sale at 21 keys, though.

Going with 23-26.