The trouble with that policy is, the seller is the ONLY person that knows where the line is between a decent offer, and "low baller". If not open to offers, you should clearly state that, or the more common phrase, "price is firm". I've seen LOTS of FBM listings that say, "no low ballers"...... does that mean "not taking offers", or "don't make an offer below the number that's my personal threshold of where low balling starts, but I'm not telling you what that number is"......
I've made offers on items that some people thought were "low ball", and often it's been a case of a seller thinking their item(s) were worth more than they are, AND I've had plenty of people make me low ball offers, in which case I either make a counter offer, or tell them they're simply way too low..... many of those offers(both the ones I've made, and those that others have made to me) ultimately ended up in sales anyway.