Sale 1:

Sold for DBD Elfin Deal (14) + > 0.5 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198120952280#!/compare/1505779200/1505865600

Buyer: https://backpack.tf/profiles/76561198057468576#!/compare/1505779200/1506038400

X = 15

Sale 2 & 3:

Sold for $28.6 and $25.99 on Marketplace.tf

https://marketplace.tf/shopitem/76561198121410799/480;5;u61/

https://backpack.tf/item/2857528098

https://backpack.tf/item/2216648930 (Woifi’s sale, I’d assume)

28.6/1.95 = ~14.66666666666

25.99/1.95 = ~13.3

X1 = 15

X2 = 13

Sale 4:

Sold for Purple Confetti Stainless Pot (14)

http://www.tf2outpost.com/trade/29340776 - No Outpost

Seller: https://backpack.tf/profiles/76561198112281935#!/compare/1503532800/1503619200

Buyer: https://backpack.tf/profiles/76561198124999895#!/compare/1503619200/1503705600

X = 14

Sale 5:

Sold for 13 keys + 2 ref

No outpost

Seller: https://backpack.tf/profiles/76561198349886701#!/compare/1502064000/1502150400

Buyer: https://backpack.tf/profiles/76561198142328142#!/compare/1501632000/1502150400

X = 13

Sale 6:

Sold for Strange Pro. KS Rust Botkiller Rocket Launcher (11) + 1 key

http://www.tf2outpost.com/trade/29305512 - B/O of 12

Seller: https://backpack.tf/profiles/76561198249553546#!/compare/1501718400/1501804800

Buyer: https://backpack.tf/profiles/76561198349886701#!/compare/1501718400/1501804800

X = 12

Sale 7:

Quicksold for 11 keys

http://www.tf2outpost.com/trade/29289824 - B/O of 15

Seller: https://backpack.tf/profiles/76561197991111981#!/compare/1500595200/1500681600

Buyer: https://backpack.tf/profiles/76561198301167028#!/compare/1499990400/1500768000

X = 11

Sale 8:

Sold in ~2 days for 11 keys

No outpost

Seller: https://backpack.tf/profiles/76561198142328142#!/compare/1502323200/1502496000

Buyer: https://backpack.tf/profiles/76561198059354111#!/compare/1502323200/1502409600

X = 11

Sale 9:

Bulk for Vivid Rotation

Seller: https://backpack.tf/profiles/76561198027150240#!/compare/1505779200/1506038400

Buyer: https://backpack.tf/profiles/76561198120952280#!/compare/1505606400/1505779200

Sale 10:

Sold for Purple Confetti Brain-Warming Wear + ~1 key in items

No outpost

Seller: https://backpack.tf/profiles/76561198301167028#!/compare/1500940800/1501027200

Buyer: https://backpack.tf/profiles/76561198036952830#!/compare/1500940800/1501027200

Seems pretty low, with a seller at 9 on the Brain-Warming Wear

Sale 11:

Sold with a couple keys for Terror-Watt Tam

Seller: https://backpack.tf/profiles/76561198059354111#!/compare/1503014400/1503100800

Buyer: https://backpack.tf/profiles/76561198112281935#!/compare/1503014400/1503100800

Should be used on the Terror-Watt

Sellers:

14 keys:

https://i.imgur.com/dqMBSkb.png - 8 days

Sales at:

11 (2x)

12

13 (2x)

15 (2x)

Sales at 11 quicksold, excluding 11.

Going with 12-15.