In Paypal scams like this, the account is often compromised, or backed by a stolen credit card. This means a month or two could go by before all the warnings are raised along the line, and then suddenly you'll be out the cash when it gets clawed back.
There are some seller protections you could take, but it would involve matching photo ID to the verified account holder name + address, but in the end it's DEFINITELY not worth the hassle.
That said, it's my understanding that Western Union is not in fact reversible. You go in to a Western Union depot and receive the money transfer as cash, full stop. Interac on the other hand has a digital paper trail, so if it winds up having been funded from a compromised account, there's a chance that it could be reversed all the way to you.
Bottom line, as Vengeance says, cash is king.