Sale #1

Sold for 10 keys 4 ref pure, rounding ~10 keys

BP Before

http://backpack.tf/profiles/76561198020364881?time=1441868400

BP After

http://backpack.tf/profiles/76561198020364881?time=1442646000

Sale #2

Sold for ~275 ref in items, rounding to ~17 keys

BP Before

http://backpack.tf/profiles/76561198118838533?time=1441522800

BP After

http://backpack.tf/profiles/76561198118838533?time=1441609200

Taking as an outlier.

Sale #3

Sold for ~8 keys pure & 4 keys in items, rounding to ~12 keys

BP Before

http://backpack.tf/profiles/76561198061228469?time=1439362800

BP After

http://backpack.tf/profiles/76561198061228469?time=1439535600

Sale #4

Sold for a Nuts n' Bolts Old Guadalajara painted green, rounding to ~12 keys

BP Before

http://backpack.tf/profiles/76561198071140072?time=1438652795

BP After

http://backpack.tf/profiles/76561198071140072?time=1438671600

Conclusion

10-12 keys seems to be the most common trading point.