Ok, let's break this down:

1st link: One 4 eerie key offer, no other interest shown in 4 days

2nd link: No offers above 5 keys in 6 days.

3rd link: Identical to 2nd link.

Also I found sellers trying to get for 6 keys or less:

http://www.tf2outpost.com/trade/5833032 (6 key)

http://www.tf2outpost.com/trade/5795004 (5 key)

The price of keys going up is probably going to stagnant most items worth several keys for the rest of the h'ween period.

How is it relevant how many are there of these?

He's just saying there's not many for sale. I agree, I bought this item about two weeks ago for around 13-14 ref, because there were a grand total of about 6 sellers and I picked the cheapest.

Now, of course, you have to find buyers at 7 keys to make this suggestion accurate and I don't think there's enough proof for that. 5-6 is perfect at the moment.

Edit: Oh yeah full disclosure: I'm biased because this hat is one of my favourites in the whole game, and there's two of them for sale on Outpost painted black already. :)