Sale 1:

Sold for 21 keys

http://www.tf2outpost.com/trade/28156809 - B/O of 21

Seller: http://backpack.tf/profiles/76561198006544512#!/compare/1462838400/1462924800

Buyer: http://backpack.tf/profiles/76561198128707636#!/compare/1462838400/1462924800

X = 21

Sale 2:

Sold for 19 keys

No outpost

Seller: http://backpack.tf/profiles/76561198202187896#!/compare/1464048000/1464134400

Buyer: http://backpack.tf/profiles/76561198104859700#!/compare/1464048000/1464220800

X = 19

Sale 3:

Sold for 18 keys

http://www.tf2outpost.com/trade/28324632 - B/O of 18

Seller: http://backpack.tf/profiles/76561198095978190#!/compare/1468281600/1468368000

Buyer: http://backpack.tf/profiles/76561198084044161#!/compare/1467244800/1468368000

X = 18

Sale 4:

Sold for 18 keys

http://www.tf2outpost.com/trade/28221760 - B/O of 18

Seller: http://backpack.tf/profiles/76561198121132234#!/compare/1464825600/1464912000

Buyer: http://backpack.tf/profiles/76561198071805624#!/compare/1464825600/1464912000

X = 18

Sellers:

19 keys:

http://puu.sh/q1fHj/126c9c7194.png - young

20 keys:

http://www.tf2outpost.com/trade/28324236 - 4 days

http://www.tf2outpost.com/trade/26399075 - 1 year? Was offered 20 in the offers section

21 keys:

http://puu.sh/q1fIL/9ca0f40538.png - 10 days

22 keys:

http://puu.sh/q1fJx/ac8c6b81f8.png - 21 days

Offers:

http://www.tf2outpost.com/trade/28290276 - 2 weeks, 5 days unbumped

http://www.tf2outpost.com/trade/28128596 - 2 months, 5 days unbumped

Sales at:

18 (2x)

19

21

Going with 18-21