What to do with very low offer on an item for sale

I either don't reply or reply with a price *above* my asking price.   I take time to fully research the market for what I sell.  I write a very detailed description and include a lot of very clear, well lit, high resolution pictures.  I set my asking prices slightly above recent average sales, but not at the extreme high end.  Some may say my prices are aggressive, but I have received my asking price or very close 95% of the time.   I recently listed an item in several online sites and received a low offer from one person.  I replied with a higher price and did not receive a reply.   The items sold at my asking price 2 weeks later.  A week after the sale, the low baller sent another inquiry reiterating their original offer.  I replied that the item was sold for full price.  Person replied and seemed to be very surprised that my item sold. 

I've done a couple thousand online transactions (not all audio) and have begun to see this type of bottom feeder on the rise. Besides rage baiters or trolls, there are now a class of folks who send out dozens of responses a day, hoping to score one and then flip it.  I get at least one for every posting I do.

Facebook marketplace has recently added a "Deliberately Low Offer" to their list of things you can negatively report a buyer for along with things like "Scam" and "Didn't Show Up".  

Ignore or at the most say no thanks. You have no obligation to them and they clearly don't care about treating you honorably. 

I see the only thing you have listed in Marketplace is a twenty-year-old, 1/2 meter pair of Cardas Golden Cross interconnect, asking $400.00.  To me, it sounds like a lot, particularly its age and short length.

" I think that if (when) I offer more of my unneeded stuff, I shall know how to manage ridiculous offers.  How would a car salesman respond if I offered $20k on an $80k car?"

Am I assuming a used car?  Some new cars lose twenty percent when you drive them off the lot.  And a used car?  The variables of what a used car would sell for are infinite.  Seems like you have little experience selling anything, but you will figure it out.  My experience has been: You never know what will sell and what sits months without a bite.  I once got an older Marantz amplifier given to me, and it had an issue. I found out that an internal fuse on one channel would go up in flames(like a torch) when turned on.  I didn’t want to deal with it, so I put it up on CL, best offer.  I even had a short video of the flameout.  After putting the ad up, I was heading out maybe a half hour later, and for some reason, I checked the ad before I left.  I had a half dozen responses, some begging me to call them back quickly. Apparently, what I thought didn’t have much value had quite a bit of value, but back then, I didn’t know much about what was what in audio.

Why allow offers if you clutch your pearls when a buy is trying to get the best price? You are doing the exact same thing. It's a negotiation if you allow offers. As a retired Navy vet whose been all over the world it's a slap in the face if you don't try to negotiate in some countries. I assume you allow other nations to make offers, unless you say will not ship overseas. You are cutting out those individuals. There are immigrants in America who grew up negotiating. Older immigrants moved here under that mindset. You're telling them to f off and it's through no fault of their own. Some need to get a grip or don't allow offers if you're so easily offended.

I'd response by saying "Here is the lowest I am willing to go: $xxx."  Take it or leave it.  But don't ask for further negotiation.