Last I heard, NADA was a retail price book, i.e. It quotes what a dealer would stick on the boat in a display lot, not the bargained price it sold for.
The BUC Book is supposed to list actual selling prices only, and you can go to their web site to get a quote on a boat's BUC vlaue for free. Somewhere in between the two, is reality.
The bottom line of course is to hand the broker a check, along with your offer, and see if the buyer accepts or rejects it. Unless the buyer has said "Don't bother to call me for under $#####" he's obligated to ask the seller if they'll take it.