OBO stands for "or best offer." It's typically used in situations in which a private owner, a.k.a. that dude with the yellow teeth who keeps grinning at you and telling you you have a purty mouth, is trying to sell to another private party, a.k.a. you, hopefully before he hogties you in his basement.
See, most of the time people who have stuff to sell know for a fact that the price they put online, or in the paper, or in skywriting over the Superbowl (admittedly not cost-effective), is a complete and utter joke with regards to the pittance they'll actually get shilled out to them for their second-hand piece of crap. This is where OBO comes in: "or best offer" is basically saying to prospective buyers, "Look, I know that I'm not going to get what I really want for this, but I'm really desperate to get some cash for this garbage."
It allows both parties to walk away with their dignity intact. The guy with the listing for that canary yellow 1987 Vauxhall gets to wait a few days to call you, hoping for a better offer from some half-blind eccentric millionaire with a passion for rusted siding, and you get to walk away knowing that you didn't succumb to the allure of buyer's frenzy - good for you, by the way - and that you'll most likely be receiving a panicked call very shortly, during which the seller will beg and plead with you to stick to your original offer. If you're feeling magnanimous, you might only shave off $50 or so, because, let's face it, he definitely isn't getting a better offer out of you.
Alternatively, OBO can stand for "one by one" with regards to professional figure skating. I didn't think that's what you wanted, but, if you are in fact a nancy-boy (no offense), please let me know so I can amend my answer.
"Or best offer"
Answered: 31 Jan '12, 07:00