My first skin suggestion, lets try it!

Sellers at 22 and 26 keys. Seller at 22 is one month mature.

SALES

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http://www.tf2outpost.com/trade/29215736 - B/O 27 keys

https://backpack.tf/profiles/76561197961066474#!/compare/1497398400/1497571200

Quickold for 18 keys pure.

https://backpack.tf/profiles/76561198094790391#!/compare/1498521600/1498780800

https://backpack.tf/profiles/76561198110832151#!/compare/1498176000/1498780800

$43 PayPal. You can't tell but you can see it was $40 in the sellers comment on his the buyer's trust page.

https://backpack.tf/trust/76561198110832151

$40 / 1.85 = 21.6. Rounding to 22.

EDIT: I added the seller and he said that he sold for about $43. http://imgur.com/nZ50nHI

So that would make it 23 keys.

Supportive SCM sale:

https://steamcommunity.com/market/listings/440/Strange%20Purple%20Range%20Sniper%20Rifle%20%28Minimal%20Wear%29

Sold for $59.24 / Keys were $2.43 at the time = 24 keys

We have:

Mature seller at 22 keys

Cash sale at 23 keys

(Invalid) SCM sale at 24 keys.

Capping at 22 because of seller.