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You're not going to find many traders that will do this. Keys are going for 6.66 ref almost exclusively over the last month or so.
What I'd suggest is go here:
Sell some of your weapons or hats for scrap (or ref for a hat) and get a key off of one of their bots.
I don't think you're going to find any takers at 6 ref and believe me, I've looked as I'm sitting in the same spot with ref haha.
Trading my last 6 Refined Metals for a 1 TF2 Key.