X-rated Bob wrote
You're entitled to put in a low bid (unless the advert says that the price is not negotiable).
The seller is entitled to say "no thanks" or to counter.
You are then entitled to decide on your next step.
Before you make an offer decide what your maximum is.
+1
If you're the seller and someone offers you less than you want, just say "no".
IMHO there is no reason to feel as if your person or sale item has been insulted by a low offer.
Likewise feel free to offer what you believe something to be worth and expect the seller to say "no" if they disagree.
And don't feel insulted by the rejection either.
Sale = willing buyer / willing seller.