According to About.com, when selling cars, "OBO" means "or best offer," which opens up price negotiations and other possibilities. "OBO" can be utilized to encourage buyers who might not otherwise feel they can afford a certain vehicle, and it can also be used to test the waters for attractive offers.
The best offer tendered for a car listed with an "OBO" is not necessarily the most money. For example, if a vehicle is listed with a price of $15,000 OBO, the seller might be willing to take less cash in a deal or perhaps take another vehicle or something else the buyer owns in a trade or partial trade. The term is also used in applications other than purchasing a vehicle, with essentially the same meaning.