People are busy creatures. If you lose their interest, you neither sell nor profit. Fear of being scammed or ripped-off. You would be surprised to know that many copywriters unknowingly contradict themselves leaving the reader suspicious, thus destroying your credibility with them. Your product should sound like the cream of the crop.
Fear is both a weakness and strength, but also a powerful selling tool.
Fear of injury, death or missed opportunity. If you sell a safety product you would use this fear to your advantage. If you're offering an opportunity, the fear of missing their chance is a strong seller. Address it in a positive manner. This creates a sense of goodwill between seller and buyer.