There is no contract without consideration. Until such time as something of value changes hands, it is merely an offer to buy.

As far as your selling fees go, you can approach your previous 'buyers' about mutually cancelling the sale (not recommended, as you have probably alienated them) or open Non-Paying Bidder disputes.

You mentioned 4-500km trips to inspect. If that was indeed the case, do you really think a prospective buyer would be prepared to travel that distance to inspect before purchase? Not knowing if the boat would still be available when they got there? Not necessarily having a way to buy the thing (through eBay, not $$) even if it was?

I suppose it depends on whether or not you really want to sell the thing. As you are obviously only looking for local buyers, maybe try one of the free classified sites.