Does a purely cosmetic flip trigger any special paperwork if no wall gets opened

Two closings and a scope of work is the core of it, but "no filing at all" isn't quite safe as an assumption, and the reason is that the triggers are local rather than tied to whether you opened a wall.

Permitting is set by the jurisdiction, and cosmetic work is usually exempt. Paint, flooring and landscaping almost never need a permit anywhere. The edges move though. Swapping a vanity is a plumbing fixture change and some jurisdictions want a permit for that, replacing a water heater usually does, and a few places require permits for re-roofing or window replacement even where nothing structural changes. Call the building department and ask what's exempt in that specific city or county, because two towns twenty miles apart genuinely differ.

Contractor licensing is a state matter and it can bite even on light work. Some states set a dollar threshold above which anyone performing work for compensation needs a license, and there are usually owner-exemptions that apply differently depending on whether you live in the property or bought it to resell. If you're hiring out the work rather than doing it, the question becomes whether your contractor is licensed and insured, and you want to see the certificate rather than take the word.

On the repeated-sale point, you weren't imagining it. There's a tax dimension, where someone buying and reselling regularly may be treated differently from someone selling an investment held long term, and how that lands depends on facts a CPA needs to look at. Separately, some states and lenders have rules about reselling within a short window after purchase, which affects your buyer's financing rather than you directly. Both of those need a licensed professional in your state, not a forum.

The piece people skip is the seller disclosure form at the exit. You'll be asked what you know about the property, including things you chose not to repair, and the content of that form differs by state.