bot.tf stats:

https://bot.tf/stats/item?def=30918&q=5&ef=Showstopper&craft=1&aus=0&ks=0

Past 90 days:

1x 19 keys (https://backpack.tf/profiles/76561198340634356#!/compare/1549580400/1549666800)

1x 17 keys (https://backpack.tf/profiles/76561198305676157#!/compare/1550876400/1550962800)

1x 18 keys (https://backpack.tf/profiles/76561198286937752#!/compare/1552176000/1552262400)

1x 20 keys (Can not find evidence of the sale, not using.)

5x 13 keys

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Marketplace.tf stats:

https://marketplace.tf/items/30918;5;u3001

Past 90 days:

1x $39.00 ($39.00 / $1.77 = ~22 keys)

1x $30.00 ($30.00 / $1.80 = ~17 keys)

1x $39.00 ($39.00 / $1.80 = ~22 keys)

1x $15.50 ($15.50 / $1.80 = ~8.5 keys)

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #1:

https://backpack.tf/profiles/76561198516017657#!/compare/1554854400/1554940800

Sold for 13 keys.

13 keys

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #2:

https://backpack.tf/profiles/76561198097271466#!/compare/1554422400/1554508800

Sold for 18 keys.

18 keys

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #3:

https://backpack.tf/profiles/76561198073611549#!/compare/1555545600/1555718400

Untraceable.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #4:

https://backpack.tf/profiles/76561198256783165#!/compare/1554422400/1556150400

Scam?

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #5:

https://backpack.tf/profiles/76561198088681558#!/compare/1556236800/1556323200

Bulk.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #6:

https://backpack.tf/profiles/76561198280975775#!/compare/1548892800/1548979200

Sold for a Hellish Inferno Taunt: The Meet the Medic (15 keys) [https://backpack.tf/profiles/76561198198896481#!/compare/1549065600/1549152000]

15 keys

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #7:

https://backpack.tf/profiles/76561198044684270#!/compare/1551225600/1551312000

Quickbuy.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #8:

https://backpack.tf/profiles/76561198118821301#!/compare/1548979200/1549065600

Sold with ~11 key for another Showstopper. Too low.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #9:

https://backpack.tf/profiles/76561198417572742#!/compare/1552348800/1552521600

Untraceable.

----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Sale #10:

https://backpack.tf/profiles/76561198264858514#!/compare/1556496000/1556582400

Untraceable.

Sales:

8.5, 13, 13, 13, 13, 13, 13, 15, 17, 17, 18, 18, 19, 20, 22, 22. Going with 13 - 17, 13 keys is too common not to include. Current sellers around 17 keys.