Can we get this straight people, it is the SELLER that requires a defence against prosecution, NOT the buyer!
If the seller is stupid enough to sell you (or your parents) the RIF then they have committed an offence and are liable to be prosecuted. Not a situation I'd ever seek to put someone in.
This particular bit of the law is perfectly clear. It might be unclear as to what actually constitutes a defence, and it might be unclear about many other things, but under section 36 of the VCRA it is an offence to sell a RIF, full stop. It then goes on to say that it is a defence against prosecution to prove the sale of the gun is for certain prescribed uses.
A buyer cannot commit an offence (unless they are under 18, then it is an offence to buy or attempt to buy) in the sale of a RIF.
There is no confusion, no ambiguity. The offence applies whether the item is sold via a business or privately.
Persuade your mum or dad to come along to a few airsoft days with you, then they will have a defence and can buy any RIF they want, and give it to you. They still cannot sell it to you (which includes payment of ANY kind, services, "mowing the lawn" "washing cars" though I guess, if it were contingent on, for example, you getting good grades at school or something similar then it would be ok.