Thanks to Torb, Ryan, & シャコタン★ブギ for the info!

Sale 1:

https://backpack.tf/profiles/76561198062621644#!/compare/1543276800/1543363200

https://backpack.tf/profiles/76561197994697258#!/compare/1543276800/1543363200

Sold with 5 keys for P Fetti Shellmet

Use there

Sale 2, 3:

https://marketplace.tf/items/183;5;u66

Sold for 17.01, 33.99 USD

Keys at 1.67, 1.64 USD

Totals are 10.19, 20.73 keys

Sale 4:

https://backpack.tf/profiles/76561197994697258#!/compare/1545177600/1545264000

https://backpack.tf/profiles/76561198268610925#!/compare/1545177600/1545264000

Sold for G Fetti Jeepcap + ~8 keys in sweets

Jeepcap Mini:

https://backpack.tf/suggestion/5c202a7644325a35526391a1

Suggestion for 12 keys

Total is 20 keys

Sale 5:

https://backpack.tf/profiles/76561198268610925#!/compare/1545264000/1545350400

https://backpack.tf/profiles/76561198065926029#!/compare/1541030400/1545350400

Sold with ~3 keys in sweets for Leak Drill Hat

Drill Mini:

https://backpack.tf/suggestion/5c20402b44325a4312578cef

Suggestion for 16.5 keys

Total is 13.5 keys

Sale 6:

https://backpack.tf/profiles/76561198048058710#!/compare/1546819200/1546905600

Sold for 20 keys

Total sales @:

10.19, 13.5, 20, 20, 20.73 keys

Seller has successfully sold,

https://i.imgur.com/3DKDIXC.jpg

Buyers up to 10 keys,

Let's go with 14 - 21 keys