You can keep only WA endemics in WA, and you can import them from the east if you wish to pay the very high import fees imposed by DEC, but there is a total ban on importing pythons into WA, even the species listed as legal. This is said to be for disease prevention reasons, but the diseases are just as likely to be carried by elapids as well, OPMV especially, so it's stupid constraint - I believe it's to prevent the possibility of carpet forms being cross-bred with M. imbricata.
THis seller is breaking the law anyway - reptiles can only be sold by licensed dealers in WA, keepers cannot sell directly to another keeper (although that was up for review - anyone know whether that has changed recently?).
Jamie