Link 1: painted pink as hell, sold for 2 ref. taking paint at half value, this should put that hat at about .66

Link 2: not explicitly useful, but a dirty hat + sweets was offered. suggests buying around average dirty hat value.

Link 3: analog to link 1, but priced at 2.33, putting the hat at about 1 ref. not sold.

because people can buy them? that's kind of a moot point. the only people who buy hats are people who don't trade. nobody would buy a hat for 5 bucks to sell it at less than a ref, when they could have gotten 2 keys with that 5$.

supply here is basically what people bought before they started trading. it is in essence a limited amount as long as the hat value is much less than the value of the currency used to buy it, because nobody in their right mind would buy hats from the store with the intention of turning around and selling them.