Does every lowball offer even deserve a reply? Any sort of reply?
What if the lowballer isn't really interested in your product per se, but makes a business / hobby out of going to all the B/S/T ads and making lowball offers on everything. If he gets somebody to sell him something at significantly less than fair market value, he calls that a win. It strokes his ego. Gives him a thrill. Then, he can turn around and flip it (probably on a different site).
I don't think such habitual lowballers deserve any reply. It's just like getting SPAM e-mail or junk mail in your mailbox at the end of your driveway. Toss it as soon as you recognize it for what it is.
If I send you a PM, and you don't reply, after being on the site, I usually send you another PM informing you that there will be no need in replying, due to you being placed on ignore. If you were standing face to face with someone would you not answer, if they asked you a question?
Just because someone "overpaid" doesn't really mean that it's worth what they paid.
I've bought many new firearms from a brick and mortar store 12 miles from my house, for way less than I see used ones priced on here.