Go to www.boatshop24.com
Owners are actually allowed to list their boats privately on that site. most other sites allow only brokers and professionals to list. plus, boatshop24 is very popular with the German and Scandinavian crowd which are your prime target if you're selling a performance boat in Europe.
Otherwise, you can go through a reputable broker. Although a reputable one will not list the boat unless he has inspected the boat. Tell me what sort of boat it is and I might be able to point you towards somebody.