The answer to this is in the question. Is it 'worth'.....?
The worth is in the mind of the purchaser....Is it what I'm looking for? Does it tick all the boxes/have all I want on it? Do I think it's worth me parting with my money? If the answer to all three questions is yes then I'd make an offer sufficiently on the low side of what I'd be happy to pay then, logically, you end up somewhere between yours and his starting points.
I think we often like what we see and are very tempted but there are similar on offer at lower prices so have a really good look around.