Travelling Shop violates the "can't beats can" golden rule generally used for interpreting board game rules. One way of getting around the rule is using "as though" wording the way MtG sometimes does.
There's no clarity issue with the card with regards to figuring out what it's -supposed- to do but it's a little less jarring to read cards if they're technically worded on point.
Really though, it seems like adding four cards to the Kingdom, which generally has the purpose of adding those cards to the supply, then immediately turning around and saying those cards aren't in the supply, isn't how you want to do it anyway. It'd make more sense to me to say "Setup: Designate four unused Kingdom card piles costing 3$-5$ as Item piles." You can tell people that it's foolish not to pull them out of the box in the rulebook, that's pretty fair game since Bard, Tournament, and Bandit Camp are also cards that have to resort to telling people it's foolish not to pull them out of the box in advance in the rulebook.
Alternatively you could do "Setup: Add four Kingdom card piles costing 3$-5$ to the Kingdom without adding them to the Supply", which is kind of what you meant, the "they aren't in the supply" seems intended as a modifier but is actually going to function as a standalone rule when you put it in a separate sentence like that.
This technical nitpicking has led me to a funny realization : since you -are- adding these cards to the supply, unlike several other fan Black Markets you can 3 pile using the Item piles. If one player does the Item thing and the other doesn't, it's possible for the player going for Items to gain unilateral control over when the game 3-piles. I don't think that's necessarily a bad thing, it seems really interesting, I doubt it will be overbearingly powerful coming from halfwoodcutter.