You have a few options. First, if you have strong doubts about your qualifications, decline the invitation. The most important thing is to be honest about your knowledge and capabilities. It is better to decline an invitation than to misrepresent yourself and waste a customer’s time.
If you believe you might be qualified, you could accept the invitation and make a note of your strengths and weaknesses regarding the topic in your response. The customer who invited you to consult will read through your comments and determine if they want to continue.
Finally, if you happen to know someone who is a better fit for the opportunity, you could refer them and earn Referral Commissions.