A horse (house, car, share of stock) is "worth" exactly what someone is willing to buy it for. If a sixteen year old horse is "worth" $2500, then what is it "worth" when it is 10, or 8, or 6?
And yes, there are plenty of motivated sellers of horses who have fine animals. The question is, do you want this horse or do you want this horse AND a years supply of food. If you don't want to haggle or risk upsetting the seller, then you've attached some $$$ figure to avoiding haggling or upsetting someone. It's your money, spend it so you feel good.
I can't say that I've bought a horse with a starting bid of less than half the asking price, I can say that for a car, several guns, and a horse trailer.
BTW, there are ways to haggle without upsetting someone. "Fine animal you've got, but I just paid my taxes and don't have that much available cash". Or "I just can't swing the $2500 right now, what with my kid in college...."