First, don't take it personally. Second, I would just reply with "my lowest price at this point would be XXX, thanks!"
I've gotten low ball offers on items, and after sending my message with my lowest, I've sold stuff. Sometimes an offer on your low limit can be the start of a negotiation. In general, it all depends on how patient you are or if the party is truly interested. Also it depends on how abundant the item is in the market.