Buying/selling homebuilts

Browsing controller.com I noticed that there is a category for
homebuilt/experimental aircraft. My limited understanding of
homebuilt aircraft is that it is an experimental plane, built for the
pleasure/education of the builder. The builder of said plane, being
the mfr, may work on it or an A&P mechanic can.

I didn't think the FAA would allow someone to legally fly an
experimental they bought; however, the number of planes on controller
indicate that this is allowed or at least not enforced. I am curious
then about a few things:

1. Are there people who just build kit/plan planes to sell them?
2. Does the FAA simply choose to ignore them?
3. What responsibilities does the builder have with the plane?
4. As the owner of a homebuilt they didn't build, can the purchaser
work on the plane or does it now become like a 172 that must be
repaired by an A&P guy?
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?