To transfer a restricted requires agreement from both parties. The person currently on record as the owner initiates the transfer, the receiver must verify and accept.
So, you transfer your firearm to Johnny Q Badactor, and he accepts. He is now the registered owner of a firearm, and refuses to pay you.
You have no recourse. You can't call the RCMP and cancel the transfer; it's done, and you no longer own that firearm. You are now in possession of a restricted firearm not registered to you.
Sure, you can go to court to get your money, but in the meantime, you either have to send the firearm along, or remain in possession of the firearm, in which case Johnny calls the RCMP and so informs them.
On the other hand, if he pays you, and you don't actually transfer the firearm, he can take you to court, but at least, as you, in theory, never initiated the transfer, nobody is breaking firearms possession laws.
If you don't trust the guy you're buying a firearm from, use an actual escrow service.