Let me Google that for you.
http://www.eaa.org/govt/ab_qa.asp
Q: I bought a partially completed project. How does the FAA handle the work already done by the previous owner (none of it commercial), and will that affect my ability to get the repairman authorization or have it certificated?
A: If you are purchasing a kit from another amateur-builder, you need to obtain evidence—logs, pictures, etc.—from the previous builder sufficient to show that the major portion of the aircraft was completed by amateurs. The FAA does count the previous amateur-builder work as amateur-completed tasks. The total of all tasks completed by all amateur builders (yourself and any before or after you) must be the major portion (that is, more than 51 percent) of the total tasks. As long as this is the case, the aircraft is eligible for an amateur-built airworthiness certificate.
Eligibility for a repairman certificate is not based on percentage of work accomplished on an amateur-built project. The eligibility for the certificate requires that the applicant was one of the builders and “show to the satisfaction of the Administrator that the individual has the requisite skill to determine whether the aircraft is in a condition for safe operations.” The FAA is not required to issue a repairman certificate to any builder regardless of the percentage of work he or she may have done on a project unless it can determine the individual has the skills necessary to determine the aircraft in question is in a condition for safe operation.
Q: If the kit is 90 percent finished by a previous owner, so I don’t do most of the work, can I still certificate it?
A: Yes, again the requirement is that the major portion is constructed by amateur(s) for their own education or recreation. However the burden to prove that the previous 90 percent of the work was done by an amateur solely for his or her own education or recreation is on the applicant. So one needs to be ready to show evidence in the forms of logs, pictures, videos, and affidavits that the previous builder was an amateur.