I've been selling these for the past few weeks on outpost http://www.tf2outpost.com/trade/8506834 and it seems like they were coming to me because I had the lowest price on the market at 2 keys.

As most of the negotiating was done on steam here are some trade history screen shots showing I was selling at 2 keys each.(Names removed for privacy reasons)

http://puu.sh/2crrP - Trading 2 for 4 keys.

http://puu.sh/2crtu - Trading 1 for 2 keys.

http://puu.sh/2cruy - Trading one for 7.6 in metal.

http://puu.sh/2crxf - Trading 1 for 4 ref and a key.

http://puu.sh/2cryI - Trading 1 for 2 keys.

http://puu.sh/2crzi - Trading 1 for 2 keys. (Spectrum sold for 6 keys)

http://puu.sh/2crzM - Trading 1 for 2 keys. (Spectrum sold for 6 keys)

http://puu.sh/2crB6 - Trading 1 for 2 keys.

http://puu.sh/2crCp - Trading 1 for 7.6

http://puu.sh/2crBY - Buying 1 for 8 ref.

There are also other people selling Sinister Staining at 2 keys each on outpost.

http://www.tf2outpost.com/trade/8627557

http://www.tf2outpost.com/trade/8248468

http://www.tf2outpost.com/trade/8289597

http://www.tf2outpost.com/trade/8823102

http://www.tf2outpost.com/trade/9058500

http://www.tf2outpost.com/trade/8481259

I didn't see anyone selling at 1.5 keys on outpost only 2 and higher, but I personally have not seen anyone buying them for over 2 keys. The only people trying to buy them on outpost are buying them at very low quicksell prices with almost no one selling to them. I've already sold 18 Sinister Stainings for 2 keys each in a mixture of keys, ref and cash.