Hmmm, i've bought this paint for 6 ref each just yesterday. The guy who was selling it said that its hard to sell it at that 3 key range. Here is his post:

http://www.tf2outpost.com/trade/5060359

He has a lot of paint and even at this low price it is not selling. You might have gotten ripped off OP.

people still are willing to pay 3 keys but not too many. i was asking for less and some guy adds me and wanted to pay 3 keys... strange

It is 3.74 euro - less than 2 keys in store.