While what has been said about the CE mark , for a European boat it's a mandatory requirement as it is to sell one there.
However there is value to the categorisation, it shows that the boat had at least been built to the basic standards that each each category requires. It also means the builder
has a properly documented production and quality control system and retains such build documentation
The ISO standards behind the scantlings, the onboard systems hatches etc. Noise
emissions, STIX, downflooding etc do ensure that you get a reasonable boat. I certainly wouldn't buy from any European builder
that wasn't able to produce a category A product. ( sailing ) . Unlike the ABYC, CE is a mandatory system where the builder and the build process has been signed off and certified by an independent approved Survey
House. Getting a Category A rating is not easy for any builder.
As to home built boats. They are exempt from such standards as long as they are not sold for 5 years after built. This only applies where the boat has been substantially built by the owner and no professional was employed. For example if the hull
was built for you then it would have to be CE marked. Most valuable homebuilts avail of the " post construction CE marking process". This allows them to seek a CE rating after the boat has been built.
Obviously only applies to EU boats
You would have to import a boat into Europe with the intention of selling it.
any boat owned by a EU resident must be CE marked irrespective of import , it must be marked to allow it to be used inside the EU , it matters not that you want to sell it