Horse buying and selling is one of the most difficult things. When I was selling a high-class pony for a fair price, a buyer 2,000 miles away made not only a low-ball offer, but wanted me to haul the pony to a state half-way between for delivery. I laughed and laughed.
When shopping, I have almost always paid full price for a nice horse. However, I have seen some horses so overpriced that I wouldn’t even look. There is one advertised right now. Cute horse doing upper level dressage work at a very young age. To me, the trainer taught it “tricks” to raise the price to an astronomical level. When people expressed shock at the price, the trainer deleted the price from the ad. I expect that horse to be on the market for a long, long time.
As others have stated, there is tactful way to negotiate price if it seems too high. The seller may be open to some wiggle room for the right buyer. However, throwing out a ridiculous low-ball offer is never a good idea.