I have sold many many cars on craigslist... and have bought many as well.. I personally don't like to offer because 1. my offer is going to be considered a low ball... or 2. I should have kept my mouth shut and offered less!! I know that everyone selling something has a number they wont go past.. that's what I ask for when I ask "whats the lowest you will take" .. makes it easier for everyone.. and yes the obo is just a start for the range that you want to sell it in... So when someone asks whats the lowest you will take.. just tell them! There has been several times when a seller wouldn't just give me a number so I said forget it...
Seems you and I think alike.
If I'm asking $1500 for something but will take $1200, when they ask I'll split the difference and let them feel like they got me for less than I was willing to take.
The one that gets me is when people list something for sale with an obo, then when you talk to them they say they're firm on the price, or say they'll need more than the asking price, or try to pressure you by saying they have 3 other guys (always 3) who're on their way over to buy it. I usually just say, "ok, you have my number if all 3 flake and you're willing to stick to your obo".