I would advise you to suggest to the vendor that the transaction be conducted through the offices of a lawyer in France, who could hold the funds in escrow (compte sequestre). They could also make sure that all of the legal i's are dotted and t's crossed, such as ensuring there is a correct transfer of title, that there are no charges on the boat for outstanding credit (garantie de non-gage), cancelling the French registration and so on.
I know that this would involve you in legal expense, but I'd say it's usually money well spent.
In any case, if this is a scam, you will almost certainly find that the vendor does not like the proposal of involving an avocat.
I used this site over a year ago. It may be legit.
Some sorry boats on there, I think they may be an agent as boats appear elsewhere too. Similar to some sights here that do similar.
I spoke to a fella who was very nice, but didn't take it further.
But who would buy a boat without seeing or smelling it?
Not me.....
PS My son is still 6 months into fighting a PayPal refusal to refund him as a seller, never mind a buyer !