If you want to use a broker, it would help to know where you are. I'm sure there are people here who can give you some good recommendations.
If you do use a broker, he/she will most likely use Yachtworld.com and maybe boattrader.com and boats.com as well.
If you want to sell it yourself, yachtworld is only available to brokers. You can list on the other two.
Trading in...while that is a possibility, most likely, not going to happen.