It used to be a thing called "verbal contract" in the old days, when two parties agreed on a sale and shook hands, no paperwork involved. It was a binding legal contract. (It might still exist legally?)
When l bought my yacht in Qld l was not asked for any deposit or signature until the owners accepted my offer.
That was
after the sea trial but before survey. It was via the brokerage, thou.
Gazumping used to be an everyday practice of shoddy operators in the eighties until it was legislated out of existence.
I strongly disagree with the practice or even just the suggestion of any deposit on a purchase of something not just yachts, when the buyer just looks at the item, before trying it.
This practice is a business trick, based on the sudden strong emotions of the interested party when they see something they might be in love first sight, and the broker or dealer is trying to catch them unawares, tying them down with a contract.
A decent seller - even a broker - would leave the buyer some room to consider a purchase before committing himself by an often hard to reverse financial commitment.
Your experience is a lesson to all, you done the obvious thing letting the issue go, however, it might have been interesting to see the consequences, had you not?