as for being able to sell them.....the same logic applies to...for example...power points, sockets, plugs, ceiling fans...the list goes on....but unless you are a licenced sparky..you cant install them...but you can still buy them.
I used to sell heaps of cast nets in my tackleshop in sydney, always with the warning to the customer that they were illegal to use in NSW waters, most of the statements in reply to that were....'we are going on holidays to qld and they reckon you need one up there'.....its not illegal to sell them....the same as drag nets....not illegal to sell them..but up to the customer to seek out the appropriate licence to use them, otherwise you could buy fish for a lifetime with the fine you would cop if caught using one without a licence.
As a word of warning....do not even THINK of using a cast net in NSW waters...I know that is one of the things the Fisheries guys and gals down there are red hot on policing.
Greg