Although we don't know if it's applicable to
this case,
this question usually comes up in circumstances
where the seller has already spent part of the deposit,
and is now concerned in case they have to refund it
to the would-have-been-buyer.
Hold that thought.
Now, while it is theoretically possible to argue
that there may be a technical legal basis for keeping the deposit,
the client-centred perspective here may be something more like
"What is it going to take to make this be over,
to get this troublesome person out of my life,
and free me up to sell the boat to someone else?"
To which the answers may well be...
1. Give them back the money, call it all off, and let them go on their way
(in the language of contract law - elect to accept a repudiatory breach).
One issue with this option can sometimes be that the seller has already spent part (or all) of the deposit.
A further issue with this option is that the seller cannot always get past the resentment and frustration they feel at letting someone "get away with it" and giving the money back.
These feelings are not really something where lawyers can help much.
Except to remind the client of the bit about making this all be over; or
2. Take them to court over either an unpaid debt, or in a breach of contract action
The issue with this option can be that the first question the court will ask is... Is there even a contract?
That's the basis of my bit above where I say
Either you sold her the boat, and she has not yet finished paying for it, or
you sold her the right to decide later whether or not to buy the boat (an "option").
The next question could then be something like
"Well, what loss have you suffered that keeping the $4000 would remedy?"
To which your answer (so far as we know from what we have here is... "Well, actually nothing.").
Then there could be a question something like...
"Why, in all the circumstances, was elected recission was not acceptable to the plaintiff?"
Given that, as far as we can tell from what we have, you can't demonstrate loss beyond mere inconvenience*
the answer is again "Well, actually, ummm.... errr... "
At which point, you are ordered to return the $4000, and you might even get to pay the other parties costs as well as your own.
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* You have not shown us here that you have lost any money.
Unrealised, unspecified, speculative, capital gain from a sale to someone else, but that didn't actually happen
is not a compensable loss.