The one and only rule to avoid scam is 'Don't pay money of any form for a job'. No serious employer need your money, they only need your service. It is only a SCAMMER that need your money to feed his hungry self and family.
Most international job forums have scam sections where scams and suspected scams are reported. I think
www.naijahotjobs.com being the number one job forum in nigeria, needs a section for scam reporting so that if anybody get an offer from a scammer or see a advert that looks scam-like can search in the scam section. I hope the adminstrator reads this.