...you are talking bordering states, which then have another set of rules.
Not for handguns at all per federal law. Handguns can't be sold across state lines period. Only from an FFL in a state to a resident of that state or third party from one resident of the state to another.
Long guns can be sold across state lines per federal law (as long as an FFL is involved), but only if the sale would be legal in BOTH states involved. Some states still have the contiguous state exception in their state laws, some don't allow it at all.