Or Best Offer. Saying that you want to sell your car for $2500 obo, means that you are wanting $2500 for it but will take the best offer below that if no one wants it at that price. $2500 would be your full asking price... if someone offered to pay you the full price, then take it! If you suspect the car might be worth more, then your asking price should have been higher.
A good strategy for selling the car would be to ask higher than the car was worth say... $3500 OBO, then settle for $2500-$3000 if you get no responses for full price.
Holding out on a man who is offering the full asking price, may send him mixed signals. He may end up thinking you aren't a serious seller.