Quoted from TomGWI:My definition of:
FIRM: this is my price. I am not going to sell it for cheaper so don't even try.
OBO: Or Best Offer
It's really not that hard to understand.
I believe people list it as firm because either that is what they have into it or think the market value for the pin is and they don't want to be bothered by the pinhead who will ask for a discount because they are looking for a deal.
I believe people put best offer because either they have it listed to high or they are just trying to get rid of a game and they will entertain offers.
Again, not hard to understand.