No dumb excuses I can remember. I've certainly had people negotiate and I have done the same. I don't get bothered by low offers and don't mind saying no. If I offer someone less than they are asking. It's not an attack on their person. It's an offer to buy a game.
Sometimes, I think excuses are people's way of trying to rationalize a lower offer that they are uncomfortable making in person.
If someone puts "Firm" in their price, I usually don't bother making an offer. It's saves us from each other. I see them as inflexible on that item and they might see my lower offer as some kind of insult. No need for all that unless it's a good deal from the beginning.