Bit of a clumsy range, definitely possible to buy for 5 keys, but I’m not sure that this can’t still get 6.

5 keys

http://www.tf2outpost.com/trade/8740251 - Selling for 5 keys

http://backpack.tf/classifieds/search/1000/11/0 - 2 selling for 5 keys

5.5 keys

http://www.tf2outpost.com/trade/8654031 - Selling for 5.33 keys

http://www.tf2outpost.com/trade/8714920 - Selling for 5.5 keys

http://www.tf2outpost.com/trade/8679591 - Selling for 5.5 keys

6 keys

http://www.tf2outpost.com/trade/8651212 - Unsold for 6 keys

http://www.tf2outpost.com/trade/8667594 - Unsold for 6 keys

http://www.tf2outpost.com/trade/7886355 - Offer of 6 keys

Definitely a cleaner and more accurate range than the current suggestion.