You assume that your strategy worked in your favor because you got the boat for "way less than asking price." Not necessarily a valid argument.
Most boats sell for well below the asking price, so I'm not convinced.
When the time comes to sell our baby, you can bet that the buyer's attitude is going to play a big role in how willing I am to negotiate, and whether or not I'm gonna throw in some or all of the thousands of dollars worth of loose goodies (spares, tools, repair supplies, charts, binos. handheld vhf, nav tools, etc, etc.) simply based on how they come across.
I might also decide to let them know what all my intended maintenance projects are, which could save them the trouble of promulgating their own punch list from scratch.
My own thinking is that since sailing is supposed to be a fun and inclusive sport or lifestyle, you ought to approach it that way even when shopping for a boat. Be respectful, fair, and when necessary firm; you might be surprised what that does for you in the long run.