First it depend how much lower it is the offer.
If it is too much lower I would reply simply that, too far from a reasonable price, I would also wait for the reaction, you may discover that the person was not really interested or it simply has no idea, or even more that it is a speculator....
If the offer it is not so much lower I would reply with a counteroffer, a little higher, and I will add it is negotiable , if the buyer it is serious you will go to sell it near your asking price. IMHO
