Counter. Sorry Crimson, plenty of time to re-suggest.

Sale 1:

Sold with <0.5 keys in items for Planets Capo’s Capper (13)

No outpost

Seller: https://backpack.tf/profiles/76561198100360306#!/compare/1496448000/1496534400

Buyer: https://backpack.tf/profiles/76561198031126990#!/compare/1496448000/1496534400

X = 13

Sale 2:

Sold with 11 ref for 11 keys

Seller: https://backpack.tf/profiles/76561198016530188#!/compare/1494374400/1494460800

Buyer: https://backpack.tf/profiles/76561198247588308#!/compare/1494374400/1494460800

X = 11

Sale 3:

Sold for 10 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198005032783#!/compare/1495756800/1495843200

Buyer: https://backpack.tf/profiles/76561198067095618#!/compare/1495324800/1495843200

X = 10

Sale 4:

Sold for Planets Elf Esteem (9.5)

http://www.tf2outpost.com/trade/29200803 - B/O of 10.5

Seller: https://backpack.tf/profiles/76561198067095618#!/compare/1495843200/1496361600

Buyer: https://backpack.tf/profiles/76561198100360306#!/compare/1495929600/1496448000

X = 10

Sale 5:

Quicksold for 10 keys

Seller: https://backpack.tf/profiles/76561198247588308#!/compare/1494460800/1494547200

Buyer: https://backpack.tf/profiles/76561198074764837#!/compare/1494547200/1494720000

X = 10

Sale 6:

Sold with 2 keys for Killstreak Australium FaN (10)

Seller: https://backpack.tf/profiles/76561198337858743#!/compare/1493510400/1493596800

Buyer: https://backpack.tf/profiles/76561198076417722#!/compare/1493424000/1493596800

X = 8

Sale 7:

Bulk for Peace Sign Bonk Helm

Seller: https://backpack.tf/profiles/76561198277588674#!/compare/1499990400/1500163200

Buyer: https://backpack.tf/profiles/76561198216561389#!/compare/1500076800/1500163200

Sellers:

12 keys:

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

https://backpack.tf/u/76561198216561389

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

https://backpack.tf/u/76561198005299438

Sales at:

8

10 (3x)

11

13

13 looks like an outlier. Excluding 13.

Excluding 8 also.

Going with 10-11.