es330td wrote:
3. What responsibilities does the builder have with the plane?

Here's one link that discusses this issue (note that it mentions the death
of John Denver in a homebuilt and a lawsuit involving a death in a
homebuilt gyroplane):

http://www.aviationlawcorp.com/content/liabhomeblt.html

5. How much can you trust a purchased homebuilt? I know they have to
get an airworthiness inspection to fly but that doesn't mean it was
built well. Is there a certain minimum level of quality a plane must
have to get the certificate?

Here's one broad overview article of buying and selling homebuilt aircraft
that briefly discusses those issues:

http://www.kitplanes.com/magazine/pdfs/0200p22.pdf