Sale 1:

Sold for 9 keys

http://www.tf2outpost.com/trade/29367044 - B/O of 10

Seller: https://backpack.tf/profiles/76561197982879578#!/compare/1505520000/1505606400

Buyer: https://backpack.tf/profiles/76561198129247117#!/compare/1505520000/1505606400

X = 9

Sale 2:

Sold for 8 keys + 14 ref (IDs match)

No outpost

Seller: https://backpack.tf/profiles/76561197999216780#!/compare/1499558400/1499731200

Buyer: https://backpack.tf/profiles/76561198362741913#!/compare/1493856000/1499731200

X = 8

Sale 3:

Sold for 8 keys

No outpost

Seller: https://backpack.tf/profiles/76561198362741913#!/compare/1499731200/1499817600

Buyer: https://backpack.tf/profiles/76561198108689065#!/compare/1499644800/1499731200

X = 8

Sale 4:

Sold for 8 keys + 10 ref to a quickbuyer

No outpost

Seller: https://backpack.tf/profiles/76561198108689065#!/compare/1499644800/1499731200

Buyer: https://backpack.tf/profiles/76561198065049657#!/compare/1499472000/1499731200

x = 8

Sale 5:

Sold for 8 keys

No outpost

Seller: https://backpack.tf/profiles/76561197979490192#!/compare/1490313600/1500422400

Buyer: https://backpack.tf/profiles/76561198095154703#!/compare/1500336000/1500422400

X = 8

Sellers:

10 keys:

https://i.imgur.com/4rHbIJR.png - 6 days (3x, could be much longer for 2 of them)

https://i.imgur.com/yF92aWl.png - 24 days

Sales at:

8 (4x)

9

Sales at 8 occur way too frequently to be ignored, even with the buyers at 8.4.

Should work similar to these suggestions:

https://backpack.tf/suggestion/59773915cf6c753b3c18dc08

https://backpack.tf/suggestion/590858fe0e2cad5453648876

https://backpack.tf/suggestion/5942f14ac440453fe742d63b

Going with 8-9.