Sale 1:

Sold for an Australium Grenade Launcher

(18 - 1) = 17 keys

http://backpack.tf/profiles/76561198073310569?time=1431500400 (May 13th)

http://backpack.tf/profiles/76561198073310569?time=1431932400 (May 18th)

Sale 2:

Sold for an Australium Black Box

(19 - 1) = 18 keys

http://backpack.tf/profiles/76561198066839342?time=1426834800 (March 20th)

http://backpack.tf/profiles/76561198066839342?time=1427180400 (March 24th)

Sale 3:

Sold for a duped Haunted Ghosts Sultan's Ceremonial

(21*0.9) = 18.9 rounding to 19 keys

http://backpack.tf/profiles/76561198097887364?time=1425628800 (March 6th)

http://backpack.tf/profiles/76561198097887364?time=1425801600 (March 8th)

Notes:

Mini for the duped Sultan: ~21

Sold for 21 keys

http://backpack.tf/profiles/76561198088401243?time=1432191600 (May 21st)

http://backpack.tf/profiles/76561198088401243?time=1432278000 (May 22nd)

Conclusion:

Sales at 17, 18 and 19, taking the final range at 17 to 19 keys. :^)