I think for the most part, I agree with the Don't Bother camp.
That said, there would be no harm in saying something such as,
"I noticed your ad for the Red Thneed in need of restoration. I'd just love to have a Red Thneed, as I recently took up Thneed Restoration, and I'd love to try to breathe new life into your Grandma's Special 1950 Thneed. However, for me, your $75 is too high, because one can already buy a perfectly functioning Red Thneed for $80.
I mean you no insult, but if by chance you find that this Red Thneed does not sell and you simply want to unload it, I'd be happy to take it off your hands for $15, as this would be a suitable restoration project price."
I can't see you going terribly wrong with something like that. I think that -sometimes- people see that they were innocently way off and then they realize, politely. On the other hand, if someone is being greedy and wheely-dealy, is it really worth your effort to bargain with them?
-C.