It makes sense to turn Craigslist into an auction system, ebay fees and hassle led to this. OBO means Best Offer,period.
If someone lowballs you (which everyone in this Business does) ignore it instead of getting into a haggling session.I assume that is the most the Buyer is willing to pay,much like setting Reserve on ebay.
Terms like "Firm, no haggle cash price,etc" can be placed on Ads and flippers still lowball or worse, wait till they are in your house and find things to lower the price or browbeat you with price police tactics. To avoid problems its best to have a good idea what the price will be before they make the trip.
Once something is agreed on I would never take a higher offer,though,thats ultimate crap.
This is the new way on craigslist,back to a pure auction system much like eBay was before they screwed up. If someone doesn't like it, move on.