In one sentence you say "duh, they are heavy of course they should be discounted". You say a heavy boat won't slow down an average sailor I disagree, a heavy boat will slow them down exactly the same amount as it would an expert sailor.
I think your definition of an average sailor differs from mine a great deal. Do you think all the current heavy boat owners will disclose that their boat is heavy when they go to sell it? If not, why not?
Anyone that buys anything new wants it to be perfect, or they want a discount. I am sure that even you can understand that.
Do you notice the boat speed difference with a few water bottles aboard?
Seller disclosure, that all depends on many things. Who is buying or selling, how old the boat is at that point and such. Do you weigh 2nd hand boats before making a purchase?
