Sale 1:

Sold for Miami Nights Company Man (in-date at 35, near-month-old seller at 31, taking 33) + 1 key

No outpost

Seller: https://backpack.tf/profiles/76561198246139142#!/compare/1500422400/1500681600

Buyer: https://backpack.tf/profiles/76561198196597527#!/compare/1500508800/1500681600

X = 34

Sale 2:

Sold for Orbiting Fire Salty Dog (29)

http://www.tf2outpost.com/trade/29208007 - B/O of 28

Seller: https://backpack.tf/profiles/76561198067575136#!/compare/1496620800/1496707200

Buyer: https://backpack.tf/profiles/76561198246139142#!/compare/1496620800/1496793600

X = 28 (Capped by B/O)

Sale 3:

Sold for 27 keys

http://www.tf2outpost.com/trade/29220188 - B/O of 27

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1497657600/1497744000

Buyer: https://backpack.tf/profiles/76561198125869453#!/compare/1497571200/1497657600

Similar issue with the compare links from sale 2 here:

https://backpack.tf/suggestion/59984072cf6c7533691b5fe9

X = 27

Sale 4:

Sold for 24 keys

http://www.tf2outpost.com/trade/29233275 - B/O of 38

Seller: https://backpack.tf/profiles/76561198125869453#!/compare/1499558400/1499644800

Buyer: https://backpack.tf/profiles/76561198081923149#!/compare/1499472000/1499644800

X = 24

Sellers:

30 keys:

http://www.tf2outpost.com/trade/29268959 - 1 month

Sales at:

24

27

28

34

34 looks like an outlier, especially with the month-old seller at 30 keys. Excluding 34.

24 is countered by quickbuyers: http://i.imgur.com/omdOty5.png - excluding 24.

Going with 27-28.