I'd say from past experience that they will try and screw you as hard as they can telling you that your trade is worthless and they will do you a favour by taking it off your hands for sweet FA.......
Your best bet with a dealer is to trade with like for like in brands......As soon as you change brands that's their opening to tell you yours is worthless.........So yeah I rate boat salesmen lower than car salesmen.
Always better doing a private sale but it is convenient to just offload and pay the difference and not have to deal with the tyre kickers.
Dan