The full form of OBO is or best offer
“OBO” stands for “or best offer”.

It is an acronym that people have been using since long before cell phones and social media existed. It’s an original acronym that’s widely known, much like ASAP. You typically see it when something is being sold. A person sets their target price, and then adds “obo” meaning they are up for negotiations on the final amount.

For example, say you’re browsing through Facebook Market and you spot a cute little patio set. You click on it, review the details, and then check the price. “$300 obo”. Well, $300 is a little more than you want to pay but it’s really cute. And they said “obo” so what the heck. You offer $220. Thirty minutes later, you get a response. Congrats. You are now the proud new owner of a patio set. Go collect your prize.

Conversation examples using “OBO” as “or best offer”.
Speaker 1: wooden swing set for sale. includes two swings, monkey bars, plastic slide, teeter-totter, and see-saw. Less than two years old. Must be picked up. No holds. $300 obo
Speaker 2: I’m interested in this item. Will you take $125?
Speaker 1: Sorry but no. How about $275?
Speaker 2: I don’t have that much. Will you do $250? That’s all I have.
Speaker 1: I will accept $250. I’ll pm you the pickup address.
Speaker 2: Okay. Thank you.