It is allowed however the same rules apply to both accounts and they're treated as one (no relists across multiple accounts etc).

What error is he getting when creating a support ticket?

Are you sure you searched for trades with the hat in the "have" list and not the "want" list?

People have widely differing opinions, especially on unpriced hats. Just because someone says an item is worth "x", it doesn't mean it will sell for that amount. Your best bet is to open a trade and say you're looking for offers. Don't rush selling, sometimes it takes weeks or even months to get a decent offer. As you receive more and more offers you'll have a general idea of what price range you're looking at.

#6 https://steamhost.cn/steamcommunity_com/groups/tf2outpost/discussions/0/558755530343215312/

It seems the behavior of the button is reversed. You can still expand the notes but only after hiding it first.

Nope. Also please don't post price checks on the forum.

They're deleted when you launch the game.

#6 https://steamhost.cn/steamcommunity_com/groups/tf2outpost/discussions/0/558755530343215312/

Which item are you trying to list?