Sale 1:

Sold for Miami Nights Headsplitter (34.5)

http://www.tf2outpost.com/trade/28841501 - B/O of 37

Seller: http://backpack.tf/profiles/76561198200324314#!/compare/1490227200/1490313600

Buyer: http://backpack.tf/profiles/76561198061440669#!/compare/1490227200/1490313600

Was used in the Headsplitter suggestion, but not to set a low or high-end.

X = 35

Sale 2:

Sold for Spec. KS Aussie FaN (10) + Pro. KS Aussie Axtinguisher (7) + Spec. KS Big Kill (6.5) + ~18 keys in skins + several other items

http://www.tf2outpost.com/trade/28847968 - B/O of 33

Seller: http://backpack.tf/profiles/76561198077875601#!/compare/1491696000/1491782400

Buyer: http://backpack.tf/profiles/76561198105624582#!/compare/1491696000/1491782400

X = 33 (Capped by B/O)

Sale 3:

Sold for 33 keys

http://www.tf2outpost.com/trade/28811390 - B/O of 34

Seller: http://backpack.tf/profiles/76561198080179568#!/compare/1488758400/1488844800

Buyer: http://backpack.tf/profiles/76561198319176396#!/compare/1488844800/1488931200

X = 33

Sale 4:

Sold with <0.5 keys in taunt for Dead Presidents Tipped Lid (30)

No outpost

Seller: http://backpack.tf/profiles/76561198186479818#!/compare/1489881600/1490486400

Buyer: http://backpack.tf/profiles/76561198077875601#!/compare/1490140800/1490486400

X = 30

Sale 5:

Sold for 30 keys in ~4 days

http://www.tf2outpost.com/trade/28850080 - B/O of 30

Seller: http://backpack.tf/profiles/76561198050518375#!/compare/1490832000/1490918400

Buyer: http://backpack.tf/profiles/76561198166545539#!/compare/1490832000/1490918400

X = 30

Sale 6:

Quicksold for 28 keys

http://www.tf2outpost.com/trade/28855988 - B/O of 28

Seller: http://backpack.tf/profiles/76561198015384426#!/compare/1490918400/1491004800

Buyer: http://backpack.tf/profiles/76561198134499784#!/compare/1490745600/1491004800

X = 28

Sale 7:

Sold for 28 keys

No outpost

Seller: http://backpack.tf/profiles/76561198107855552#!/compare/1488326400/1488585600

Buyer: http://backpack.tf/profiles/76561198091836128#!/compare/1488412800/1488585600

X = 28

Sale 8:

Sold for 27 keys in ~9 days

http://www.tf2outpost.com/trade/28760730 - B/O of 35

Seller: http://backpack.tf/profiles/76561198176860775#!/compare/1486598400/1486684800

Buyer: http://backpack.tf/profiles/76561198107855552#!/compare/1486339200/1486684800

X = 27

Sale 9:

Quicksold for 27 keys

http://www.tf2outpost.com/trade/28846786 - B/O of 27

Seller: http://backpack.tf/profiles/76561198073425121#!/compare/1490486400/1490572800

Buyer: http://backpack.tf/profiles/76561198050518375#!/compare/1490486400/1490572800

X = 27

Sale 10:

Quicksold for 26 keys

http://www.tf2outpost.com/trade/28836645 - B/O of 34

Seller: http://backpack.tf/profiles/76561198278502699#!/compare/1489708800/1489795200

Buyer: http://backpack.tf/profiles/76561198041314021#!/compare/1489708800/1489795200

X = 26

Sellers:

37 keys:

http://i.imgur.com/QysCb5K.png

http://backpack.tf/u/76561198105624582

Sales at:

26

27 (2x)

28 (2x)

30 (2x)

33 (2x)

35

Excluding sales at and below 28, as the majority of them sold in a short amount of time.

Going with 30-35.