What are we talking about in terms of Real $ ?
I don't think you owe anything. But, I guess you have to ask yourself if you think you might need/want to call the PO for information about the vessel in the future and what value would there be to him taking your calls. Would you have signed the contract if box 10-a were checked?
I might propose a 3 way split. The seller, the broker and you each pay a 3rd..of the bill.
If it's a grand, then that's $333.00 apiece. Let him deal with the broker, and make sure you can verify that the tax bill gets paid.
It keeps the lawyers out of it. Spreads the pain equally and potentially preserves your ability to have good future dealings with both the broker and the PO.
Or you can simply, walk away.
When I purchased my last house, both brokers took a cut in their commission to make the deal happen.
imo...The broker screwed this deal up, so they do have skin in the game if they want a good reference.
As a practical matter, the "Tax" is likely between $500 and $750. Despite the protestations of others (posts above) in re: prorations of taxes, while we lived there, we have bought and sold 4 boats in California and, in each case, prorated taxes as of the date of sale. That said, think of this. In each case after we sold a boat, for weeks if not months afterward, we/I discovered items that belonged to the boat we sold and I would pack them up and ship them off to the buyer against the possibility that he might need them. Likewise, when we bought our last boat--that we have now--for months after we closed we discovered things aboard the boat that the seller certainly couldn't have intentionally included in the sale. A (very nice/expensive) Standard Horizon hand held radio, a very costly hand bearing compass, a collection of navigation tools from the US Navel Academy, Cerca 1965, (the Seller was a retired Captain in the Navy); video tapes of family vacations etc., etc. etc. We duly packaged these up and sent them off to him. Likewise, in his garage, he found/had many spares and parts from the yacht that, over time, he packaged up and sent off to us, including many parts that are now all but impossible to find. Had we parted on less than amicable terms, or with ill will, does anyone think these exchanges would have continued/happened? I don't think so.
For the relatively little amount that is involved I think the OP/Buyer is a fool for not making an accommodation for the Seller as I suspect in short order he/she will need information/help from the Seller. Taking the FU Very Much approach most certainly has no positive payoffs. But, really, for a few hundred bucks, is the "bad karma" worth it? If that amount is a major issue for the OP/Buyer, he/she is in for a serious wake-up call as to the cost of yacht ownership, particularly in the Peoples Republic of California.