Any business transaction that involves Nigeria is normally a scam - it's recognised by the UN as one of the most corrupt countries in the world.
Note that just because the buyer claims to be in Canada does not mean that is where he is actually located. You can use the mail headers to determine the originating IP address, and then
http://www.ripe.net to see whom the IP address is assigned.
Any business transaction that involves over payments and requires you, as the seller, sending money to somone else is normally a scam, and definitely a scam if Western Union/ Moneygram or any other untraceable cash transfer system is involved.
The transaction involving Mum Donald is definitely a scam; Rolland Johnson is probably a scam, but it's too early to say for Eric Sylvester and Bill Jeff. Ask for payments either via Western Union, or cash in hand in person when they come to collect the puppies. No genuine buyer is going to object to paying you cash in person for puppies - people tend to want to see the animal in the flesh before deciding to buy it.