I think that judging whether or not to consider a pin based on the inclision of 'firm' or 'OBO' in the ad is very limiting. Different people mean different things when using these terms but "I'm a jerk" or "I don't know what I have/I'm desparate" are limiting descriptions. I put out the last Genie as OBO, perhaps that's why you didn't make an offer
Seriously though, I use OBO often meaning I might negotiate. I haven't used firm yet but I'd like to think if I did I'd mean it, that is what I'll take minimum. No more, no less. I'll leave the interpretation for two things: the price and the actual machine.