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

As a buyer, I would never make such an offer. If a seller overvalues an item by 3-400% I don’t bother. It’s probably a good idea to note that “only reasonable offers will be considered.” Then there’s no need to reply to a lowball offer, even if the bidder thinks it’s reasonable. People are weird. I once had a guy offer me 25 cents for an item priced at 50 cents at a garage sale. He was astonished when I told him he could just have it for free. 

I try to always give a response no matter how low the offer is.

I also believe (as noted by others) that your current price is a little aggressive.

Good luck with your sale. 

Depends on if you've looked at comparable sales in the 90 days.  Are you priced appropriately?   Generally, if you're priced a lot too high, people just ignore it.  

How long has it been listed?  If it's more than 30 days, it might be a sign you're too high.

I'd check all the online auction places including ebay.

If your item is accurately priced -

"Thank You" ... and nothing more, would cover it.

@retiredaudioguy - A couple of other points:

Your ad title, "Cardas Golden reference interconnect" is incorrect as the cables shown in the image are in fact Cardas Golden Cross.  You have it correct in your text but you should fix the title.

Regarding pricing. I just purchased a pair of 0.5M long Golden Reference ICs with XLR connectors for $300, so your potential buyer's offer was probably not out of line.  Also, if you look on hifishark (link) at all former sales of CGC ICs at 0.5M with RCAs, you will see that five pair sold between 2023 and 2018 at prices ranging from $280 to $225.