From the website www.howstuffworks.com:
Car Selling Ad Terms*
OBO - This means "or best offer," which tells the buyer that you are willing to consider offers below the price you listed.
Source: Edmunds.com
Come on guys, I don't care if you are from Neptune, "OBO" means an offer LOWER than the asking price. A seller doesn't need to say "OBO" for a buyer to assume he will take a HIGHER offer than the ask -- every seller is willing to do that.
But let's not lose sight of the point that we all seem to agree on: a buyer whose first and only communication with a seller is "WYBP" has a high likelihood of being ignored -- as well he should be, b/c this response indicates a lack of seriousness and a single-minded focus in disadvantaging the seller. At the very least, a "WYBP" message might be accompanied by something like "I am really interested and would be ready to pay quickly if your best price is in my range." At least then there seems to be a point to the negotiating tactic.