Sale 1:

Sold for 26 keys

http://www.tf2outpost.com/trade/28086614 - B/O of 25, Seller confirms sale (legend)

Seller: http://backpack.tf/profiles/76561198045309178#!/compare/1461024000/1461196800

Buyer: http://backpack.tf/profiles/76561198043913481#!/compare/1461024000/1461110400

Ignoring B/O because full pure sale.

X = 26

Sale 2: (Dupe)

Sold for 24 keys + 10 ref

Seller: http://backpack.tf/profiles/76561198063638461#!/compare/1459036800/1459123200

Buyer: http://backpack.tf/profiles/76561198070558403#!/compare/1459036800/1459123200

X = 25

Sale 3:

Sold for 24 keys

http://www.tf2outpost.com/trade/28143603 - B/O of 24

Seller: http://backpack.tf/profiles/76561198264065363#!/compare/1463443200/1463529600

Buyer: http://backpack.tf/profiles/76561198096847900#!/compare/1463356800/1463529600

X = 24

Sale 4:

Sold for 20 keys

http://www.tf2outpost.com/trade/28112111 - No B/O, was offered 21 keys

Seller: http://backpack.tf/profiles/76561198053967778#!/compare/1462060800/1462233600

Buyer: http://backpack.tf/profiles/76561198264065363#!/compare/1461801600/1462060800

X = 20

Sale 5:

Sold for Dead presidents Tree?

http://www.tf2outpost.com/trade/27965463 - B/o of 22

Seller: http://backpack.tf/profiles/76561198217056368#!/compare/1458198000/1458284400

Buyer: http://backpack.tf/profiles/76561198125689461#!/compare/1457251200/1459036800

X = 18

Sellers:

25 keys:

http://puu.sh/p1MVX/60f0596ff2.png - 6 days

30 keys:

http://www.tf2outpost.com/trade/28011398 - 1 month

Buyers at 18 and 19 keys:

http://puu.sh/p1N7E/3a97d0b174.png

Sales at:

18

20

24

25

26

18 is excluded due to buyers

20 seems low, as the owner was offered 21 pure right after selling

Going with 24-26