Sale 1:

Sold for 16 keys

http://www.tf2outpost.com/trade/28874525 - No B/O

Seller: https://backpack.tf/profiles/76561198192602363#!/compare/1492819200/1492905600

Buyer: https://backpack.tf/profiles/76561198067295068#!/compare/1492905600/1494547200

X = 16

Sale 2:

Sold for Blizzardy Storm Swagman’s Swatter (Mini a: t 14.5 below) + 1 key

http://www.tf2outpost.com/trade/29181028 - B/O of 15

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1495929600/1496016000

Buyer: https://backpack.tf/profiles/76561198190714323#!/compare/1495929600/1496016000

http://imgur.com/a/nGjYN

X = 15 (Capped by B/O)

Sale 3:

Sold with 0.33 ref for Starstorm Insomnia Plug-in Prospector

No outpost

Seller: https://backpack.tf/profiles/76561198190714323#!/compare/1496016000/1496102400

Buyer: https://backpack.tf/profiles/76561198089703403#!/compare/1495843200/1496016000

Can be used on the Prospector.

Blizzardy Swagman Mini:

Sold for 15 keys in under 24 hours

No outpost

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1495929600/1496016000

Buyer: https://backpack.tf/profiles/76561198125983657#!/compare/1493510400/1496016000

http://imgur.com/a/T95st

X = 15

And sellers for ~14 keys

http://i.imgur.com/LVyem2c.png

Going with 14-15.

Sellers:

14 keys:

http://i.imgur.com/YQn3ezq.png - 2 days

https://backpack.tf/u/76561198184814974

16 keys:

http://www.tf2outpost.com/trade/29233433 - 1 week

https://backpack.tf/u/76561197964171516

Offers:

http://www.tf2outpost.com/trade/28347410 - 11 months

https://backpack.tf/u/76561198091522196

Sales at:

15

16

Seller at 14 is a bit too young to include.

Going with 15-16.