So someone's come in with an offer - great! The bad news is you feel it is insultingly low! Don't get personal. Remember it's a start point and it's your big opportunity to open negotiations. Of course buyers will want to try to buy your house fo the lowest price possible! They know they can always go up, but they can't go down. It's not personal as far as they are concerned, it's a business transaction.
So you react by coming back with a counter offer.