That's exactly what happened me few times. I ask the seller many times to say his price but he always says "offer".
Then i make offer and right away he says "lowball noob" or so. All i've said to that kind of sellers is that i'll go find another seller and that they have no idea how to have good customer service and how to be polite to potential buyer.. I think those people can even have a lack of some serious social skills.