There are always people looking to get a steal and unfortunately, that means low ball offers. Whenever I list BIN's with BO, usually half of the listings that get activity, I get some ridiculous offers...it comes with the territory of giving people an option to make your own price up. I will say that some people do get steals this way, so my advice is not to let it bother you and just respectfully decline and move on. And no, you're not missing anything or doing anything wrong
