I think Pip started this thread based on another thread where a $2700 obo taxi was for sale. I commented that when I see something like that (OBO), I tend not to even respond to the ad or make an offer as the seller clearly thinks that his game is worth the price he's asking.
The way I perceive OBO is that the seller believes his item is valued at the price he's asking but may come down a bit, hence the OBO. However, if the prevailing rate for a taxi is $1200 and you make that offer, I feel pretty confident that you're going to get the "you lowballing son of a gun" response.
In the case of the taxi, the seller indicated that he would entertain offers but with his asking price well above what most believe a taxi would sell for, OBO really doesn't mean anything in my opinion.
The wording of the ad doesn't really affect my response. If it's priced fairly and it's a game I'm looking for then I'll respond. I don't mind negotiating so whether it's firm or OBO doesn't really matter. The asking price does.
Dave