Sale 1:

Sold with 2 keys in items for Orbiting Planets Yellow Belt

No outpost

Seller: http://backpack.tf/profiles/76561198045580380#!/compare/1461628800/1461801600

Buyer: http://backpack.tf/profiles/76561198168906972#!/compare/1461715200/1461801600

X + 2 = 20.5

X = 19 (Rounded from 18.5)

Sale 2:

Sold for Circling Heart Magistrate’s Mullet

No outpost

Seller: http://backpack.tf/profiles/76561198080179568#!/compare/1461369600/1461456000

Buyer: http://backpack.tf/profiles/76561198045580380#!/compare/1461283200/1461456000

X = 19

Sale 3:

Appears to have sold for 18 keys + ~1 key in cosmetic/metal

No outpost

Seller: http://backpack.tf/profiles/76561198027703408#!/compare/1460505600/1460678400

Buyer?: http://backpack.tf/profiles/76561198064850671#!/compare/1428649200/1429858800

X = 19

Sale 4:

Sold for 18 keys

Seller: http://backpack.tf/profiles/76561198019163839#!/compare/1465171200/1465257600

Buyer: http://backpack.tf/profiles/76561198261343250#!/compare/1465171200/1465344000

X = 18

Sale 5:

Sold with ~5 keys in items for Max’s Head

Seller: http://backpack.tf/profiles/76561198151387942#!/compare/1464652800/1464998400

Buyer: http://backpack.tf/profiles/76561198092471230#!/compare/1464912000/1464998400

X + 5 = 23

X = 18

Sale 6:

Sold for 17 keys

http://www.tf2outpost.com/trade/27835578 - No B/O, never re-listed

Seller: http://backpack.tf/profiles/76561198012613726#!/compare/1464652800/1464739200

Buyer: http://backpack.tf/profiles/76561198019163839#!/compare/1464652800/1464739200

X = 17

Sale 7:

Sold for 16 keys kinda quickly

http://www.tf2outpost.com/trade/28172182 - No B/O

Seller: http://backpack.tf/profiles/76561198168906972#!/compare/1463356800/1463529600

Buyer: http://backpack.tf/profiles/76561198066609212#!/compare/1463443200/1463529600

X = 16

Sellers:

18 keys:

http://puu.sh/pKaEy/0251d966d8.png - 25 days

19 keys:

http://www.tf2outpost.com/trade/28289003 - 2 days

Offers:

http://www.tf2outpost.com/trade/28190754 - 1 month, declined 16

Sales at:

16

17

18

18

19

19

19

16 sold a little too quickly to be in the range, and another seller declined 16. Excluding 16

Going with 17-19