Sale 1:

Sold for 16 keys in ~a day

No outpost

Seller: http://backpack.tf/profiles/76561198075970209#!/compare/1488240000/1488326400

Buyer: http://backpack.tf/profiles/76561198286341212#!/compare/1488326400/1488412800

X = 16

Sale 2:

Quicksold for 14 keys

http://www.tf2outpost.com/trade/28805035 - B/O of 17

Seller: http://backpack.tf/profiles/76561198119409653#!/compare/1488153600/1488240000

Buyer: http://backpack.tf/profiles/76561198075970209#!/compare/1488240000/1488326400

X = 14

Sale 3:

Quicksold for 13 keys + 12 ref

http://www.tf2outpost.com/trade/28825691 - B/O of 16

Seller: http://backpack.tf/profiles/76561198179561250#!/compare/1489190400/1489363200

Buyer: http://backpack.tf/profiles/76561198147423168#!/compare/1489104000/1489276800

X = 13

Sale 4:

Bulk for Burning Phantom

Seller: http://backpack.tf/profiles/76561198135747549#!/compare/1492992000/1493078400

Buyer: http://backpack.tf/profiles/76561198060375373#!/compare/1492992000/1493078400

Sale 5:

Some bulk sale

Seller: http://backpack.tf/profiles/76561198286341212#!/compare/1488412800/1488499200

Buyer: http://backpack.tf/profiles/76561198120672630#!/compare/1488412800/1488499200

Sale 6:

Some more bulk

Seller: http://backpack.tf/profiles/76561198116562186#!/compare/1489104000/1489190400

Buyer: http://backpack.tf/profiles/76561198018719204#!/compare/1489104000/1489190400

Sellers:

14 keys:

http://i.imgur.com/Ek6lkot.png - 1 day

http://backpack.tf/u/76561198344933095

15 keys:

http://i.imgur.com/IzxZaKE.png - 10 days

http://backpack.tf/u/76561198107520045

http://www.tf2outpost.com/trade/28903869

http://i.imgur.com/XqKZuCR.png - 12 days

http://backpack.tf/u/76561198067575136

http://www.tf2outpost.com/trade/28889924

16 keys:

http://i.imgur.com/BWMi37K.png - 16 days

http://backpack.tf/u/76561198054758757

http://i.imgur.com/udzTp72.png - 3 days

http://backpack.tf/u/76561198002757046

http://www.tf2outpost.com/trade/28904619

17 keys:

http://i.imgur.com/fnOaijx.png - 2 months

http://backpack.tf/u/76561198192602363

http://www.tf2outpost.com/trade/28874523

18 keys:

http://www.tf2outpost.com/trade/28729867 - 3 months

http://backpack.tf/u/76561198023051867

19 keys:

http://i.imgur.com/fvsHF3z.png - 3 months

http://backpack.tf/u/76561198173027559

http://www.tf2outpost.com/trade/28856272

Offers:

http://www.tf2outpost.com/trade/28895419

http://backpack.tf/u/76561198045493149

Sales at:

13

14

16

All sales quicksold, however, they were 2 months ago. Many more sellers have appeared on the market, and this item is now oversaturated with sellers.

Buyers at 13. Excluding 13.

Excluding 14 also. Current seller at 14 is pretty young.

While 16 ‘quicksold’, there are many mature sellers at and lower than its current price. Excluding it would be a bad idea, and there are no sales at higher either.

Going with 15-16.