One significant problem. How does the seller know that are are actually authorised to purchase the new rifle? If he checks your FAC and it shows you have authority to acquire a rifle of whatever calibre, say .22, then he checks table 1 and can see that you have a .22 listed there, but you've actually just sold it to someone the day before, then how does the seller confirm that is actually the case and that you're not purchasing beyond your authority?
Unless we have access to a fully online/digital system I cant see this ever working.