I've owned this 7 times, I must love them...

Sale #1:

Sold for a Steaming Milkman (2 years outdated at 16 keys)

http://backpack.tf/profiles/76561198069810035#!/compare/1471824000/1471910400

http://backpack.tf/profiles/76561198002757046#!/compare/1471737600/1471824000

Not using

Sale #2:

Quicksell for 11 keys

http://backpack.tf/profiles/76561198021788116#!/compare/1471478400/1471564800

Sale #3:

Sold for a Specialized Australium Stickybomb Launcher (15) + 5 ref

http://backpack.tf/profiles/76561198125791160#!/compare/1470614400/1470700800

http://backpack.tf/profiles/76561198102170771#!/compare/1470614400/1470700800

Rounds to 15 keys

Sale #4:

Sold with a Sp. Strange AWPer (~6) for a Cloud 9 Rag (1 year outdated at 21 keys)

http://backpack.tf/profiles/76561198102170771#!/compare/1471132800/1471219200

Not using

Sale #5:

Sold for a Planets Soldier's Slope Scopers (17)

http://www.tf2outpost.com/trade/28464604 B/O was 15 keys

http://i.imgur.com/FJ8Dmse.png

http://backpack.tf/profiles/76561198187948905#!/compare/1472947200/1473120000

http://backpack.tf/profiles/76561198080179568#!/compare/1472860800/1472947200

Sale #6:

Sold for 14 keys

http://www.tf2outpost.com/trade/28309271

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

http://backpack.tf/profiles/76561198080179568#!/compare/1467763200/1467849600

Seller:

http://www.tf2outpost.com/trade/28441218 13 keys

Support:

~4 months ago

http://www.tf2outpost.com/trade/28168661

http://i.imgur.com/7HSrVJS.png

Sold for 15 keys

http://backpack.tf/suggestion/5796fbefc440457f4673ca63

Also a sale for 3 keys + C9 Pipe (12.5 keys) here

Conclusion:

Sales are at 11, 14, 15, and 15, with a seller at 13 keys. 11 keys was a quicksale. Range should be 13-15 keys.