If the boat is on the hard and the dealer needs to launch it, it may be reasonable to ask for the costs to launch the boat, but there is more to it than that. It may be reasonable for you to indicate serious interest by providing a deposit and offer?
The way we did it years ago was to take a good look at the boat and make an offer and deposit. We conditioned the sail on a successful survey and sea trial.
In the end we had the owner fix some really minor things, like the GPS, but went through with the purchase.
In any case you don't want to buy a boat without a proper survey and sea trial. To get that you may have to be serious enough to make an offer and deposit.