OBO does not mean what I think you think it means (shameless Princess Bride steal). For it to be a true OBO, it has to be the best offer for me. Not necessarily for you. Good OBOs are often mutually beneficial, but they certainly aren’t going to benefit the buyer more than the seller. Sellers are looking to sell, not giveaway.
I’m convinced that most people on CraigsList have no idea what OBO really means. No, it is not you offering me your junk for my used car. Nor does it mean I’ll consider trades for your old PC, (mostly worthless) “rare” coins, stamp collection, or your run down car. I’m trying to sell my run down car, why would I want yours? Obviously, the simple solution is to just remove OBO from future postings. But I’d rather come up with some rules for responding to an OBO.