The buyer violates 18 USC 922(a)(3) and the seller violates 18 USC 922(a)(5).
The (b)(3) exception is only for long guns purchased from FFLs.
If the states' in quesiton have contiguous sales laws, they mean basically nothing. Those laws were to allow the (b)(3) exception sales before a 1986 amendment to the GCA, back then out of state long gun sales were limited to contiguous states FFLs, only if state laws specifically allowed the sales. The 1986 amendment did away with that, so now out of state sales of long guns from FFLs are no longer limited to contiguous states.