IMO, It's seems to be a sham phrase now. These days OBO to many sellers just means they may (or may not) consider an offer lower than listed price. Use to was, if you were selling an OBO item you just wanted it gone by a certain time.
You all are saying the same thing! Same message! OBO means you may sell it for less if you don't get your asking price. FIRM means don't come around and offer me less, that is not an option. Why are people trying to make more out of OBO than it is!