Accepting quality offers and counter-offering the others (even the "bad" ones) leads to an overall better customer experience and a higher sales volume. Why completely stop the negotiation conversation with your shopper, when they may accept a higher offer that still yields a quality discount? We consider that a win-win!
Besides responding to all your offers in a timely manner, we highly recommend countering your low-ball offers instead of rejecting them. The only time an order should be rejected is if there are inventory issues or the customer requests it.
Even if you consider low-ball offers offensive, we suggest countering offers for a short time to see if they yield any success. Doesn't hurt to try, since you would be losing that potential sale by rejecting anyway!