I've done this
Since I bought a kit started by somebody else, I have spoken extensively to Vans, EAA, and FAA on this subject. EAA's website was not helpful at all on the specifics of this issue, and two people in the same office gave me conflicting advice. Cynthia at Vans was VERY helpful in sorting it all out.
You need the previous builder's logs and a statement from the builder stating that his or her work was done for recreation and education. You will need to have a (non-FAA) bill of sale to document the purchase, and Vans will need a copy of this in order to transfer the builder number from the seller to you.
You do NOT need to fill out an FAA bill of sale unless Vans issued one to the prior builder (typically occurs when firewall forward kit is ordered if I remember correctly). If Vans issued a bill of sale, then the chain of ownership needs to be established between him (her) and you. Otherwise, if Vans has not yet issued an FAA bill of sale, you are just purchasing a bunch of parts, not an aircraft, and the FAA bill of sale will just confuse the FAA. Cynthia at Vans can tell you whether the FAA bill of sale was issued or not.
Highest Regards,