I very seldom make offers on cards I'm buying either as it's not too hard to find the cards I want priced reasonably, but I suppose it depends on what you're buying.
On the flip side, my prices don't have a lot of room for negotiation - I list it with pretty much my best price. I would say I have rejected about 75% of the offers I have received. But the classifieds are littered with sellers offering to accept offers at 50% of list price, and I've come across too many cards that are way overpriced to assume it is anything other than a strategy to work off of offers received.
I think we're on the same wavelength. I don't think I'd make an offer on a card that was already deeply discounted or was the least expensive of its kind. I am thinking that I'd consider making a respectable offer on a card that wasn't discounted much.