Sellers for 5 keys are nowhere to be found. If there are any, they are most likely pounced upon. (eg, thatotherchigga's comment)

Bought at 6 keys:

http://www.tf2outpost.com/trade/12449656 (sold out)

http://www.tf2outpost.com/trade/10151594 (soon to be sold out)

http://www.tf2outpost.com/trade/12414015#p40843438 (I bought all of them)

http://www.tf2outpost.com/trade/11920635 (1 sold)

http://www.tf2outpost.com/trade/11362388 (rejected 5 keys)

Sold for 7 keys:

http://www.tf2outpost.com/trade/11137896 (sold out)

http://www.tf2outpost.com/trade/11491489 (sold 3)

http://www.tf2outpost.com/trade/8902588 (sold alot, but most likely not all at 7)

http://www.tf2outpost.com/trade/12025910 (Willing to overpay @ 7)

Unsold at 9 (for balance reasons):

http://www.tf2outpost.com/trade/11192597

http://www.tf2outpost.com/trade/11678772

I myself have successfully sold 4 @ 7 keys

http://www.tf2outpost.com/trade/11882056

Let me know if you want inventory history screenshots