Golden rules:
1. do NOT send gun until receiving money
2. when receiving money, only use EMT transfer through online banking, that way when you receive the money it is like cash, dont use COD or cheque(least reliable) because it can bounce
steps for transaction:
1. initiate transfer (call firearms center) and obtain reference number
2. Supposedly you shoulnd't give the reference number to the buyer until you've been paid, but typically I give the number so that the buyer has peace of mind knowing the gun is being transferred to him (besides, you still physically have the gun, what's he going to do)
3. once the transfer has been completed and the money safely received, send the gun to him via canadapost parcel (ground only) if it is a restricted firearm, you have to ship it like you are taking it to the range, gun disabled via some kind of lock (zip tie works) and in a securely locked container
Hope this helps. Note: these are only based on my experiences and opinions, others may have different opinions