Yachtworld listings are placed only by brokers. A broker therefore sat with a seller and set a price. It may be realistic or not. Not all sellers are realistic and some brokers will let a high price fly just to let the seller get some additional time then come around to lowering the price. A broker also can access the sold prices that the public can not see. They may encourage the seller in a direction. Not all sellers can be encouraged. Smart sellers sell their boats quickly for a good price.
There is no rule of thumb to prices you see advertised and what to offer. People that expect to sell their boat will set realistic prices. Do your own reasearch and you can use yachtworld to examine asking prices. If the boat is popular you'll find a common thread to the prices based on all the things you might consider important. Never offer more than you can afford.
It's OK to offer a lower price than asking, but with that comes negotiation. Negotiation is the point where both parties can agree. A hard but earnest negotiation may yield you the best price, but at that point you don't come back from the survey and demand every dime all over again. You set expectations. Personally I prefer to set my offer price and say I won't beat you up when the survey comes back unless it has serious issues. It's Ok to tell a seller you can't afford the asking price based on what you have researched and what you may need to do to the boat.
You can honstly negotiate a price without insulting the other side. Find out all you can before you sit down and preform some calculations and see really what the basis is for your asking price and the sellers offer price. Talk about it honestly. If I offer a boat at $50,000 and you present an offer for $5,000 I might dimiss it and ignore you unless you could show me how the boat is a wreck and wasn't worth that much. Personally, I would avoid boats that seem to be offereed too high for no reason. It may mean there is no negotiation possible.
Sometimes the deal can not be made. It might be your fault or the seller. Not every boat is a deal for you. So you just have to uinderstand what deals to go after and which ones to walk away from. Never offer a price you won't follow through with. Sellers want honest buyers too. Being nice won't hurt. I would give someone I liked a better deal just because I like them. That is more true than you might imagine for many people.