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

I just say "no"

That one word can convey many different sentiments, for example it could mean:

- no, don't waste my time with your games

- no, go take a hike

- no, why do I even bother responding to guys like you

- no, WTF

- no, can you sense the annoyance

You get the idea

That's pretty much the same sentiment.

when the offer is super low, I usually play along and respond with an even lower counter. 

Why get worked up over this?  There are plenty of unreasonable sellers also, which are attempting to get close to new prices or well above fair market value.  Do you email each seller also just to prove a point?  It's your choice to engage...or not.

Let's assume your asking price was in line with fair market value.  (For FMV I prefer to use what TMR would pay me plus 10-15%) FWIW I'm a frequent seller and I can tell you from my experience is that the market for used gear is mush right now. 

If you get a stupid offer that's an insult please consider that some buyers are not WASPS and in their culture an opening bid is how the party starts and not a slap in the face.  I sold to an Indian gentleman once that turned out to be a great buyer once we got past his ridiculous initial offer. 

I typically respond with a polite counteroffer of 3-5% off my ask. Once the dialogue gets going you have a good shot at an acceptable deal.