if someone says "no offers, firm price" in their ad, and then I PM a low offer, I'd understand why they'd be annoyed.
More often than not it just says a price, so obviously you'd offer lower than that.
If someone asks me for my best offer, why they hell would I actually tell them what it is? If I say to a seller "what's your best price" he's not going to tell me the absolute minimum he will take.
Some people just take things personally