How do I know if this is legitimate?

I have an opportunity to purchase two signed guitars. One is signed by the members of Led Zepplin and the other signed by members of the Rolling Stones. Both are appraised and come with certificates of appraisal. They are each appraised around $3,000. The seller is asking much less. They also come with certificates of authenticity. What should I look for to know if this is fradulent.