You have a number of options:
which has the largest selection of boats. However, beware that a number of listings are duplicated on this site, so browse carefully.
which is Yaaba Assoc. backed MLS system with good enforcement.
which gets less pr but has a good selection.
4. New mls called www.usedboats.com
which is trying to challenge yacht world.
5. Boattraderonline.com and YachtTraderonline.com which offer a mixed bag of results. Close association with Yachtworld now.
INDEPENDENT MULTIHULL SITES
- world's largest exclusive retailers of catamarans. Specialize in power and sail catamarans from 30ft up to 150 feet. Also a good resource for news
- a site with a long and reputable reputation in the multihull industry with close ties to www.catamarans.com.
There are other independent sites that will help you in your quest but the above two hold the most catamaran listings.
They also have a first-time buyer and seller guide you may find useful.
I hope the above helps you make an informed decision