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

Also depends on how long the ad has been on, how desirable the item is, and how eager you are to sell..

These are bottom-feeders looking to make quick flip profit off the backs of a desperate enthusiast (what they hoped you were), while contributing nothing positive to the hobby. You simply ignore & block them, now and forever :) 

I bet they don't accurately report & pay their taxes here, either. Though that will probably be harder to conceal now that the likes of Paypal issue an annual 1099K.

I will not entertain them under any circumstances; even if they come back with a "reasonable" offer, I want no dealings with such animals. 

It's up to you to specify the terms of your sale.

"Price is firm.' (no offers considered) for example.

"Lowballers will be politely ignored." etc.

I would like to sell 90% of my audio collection, locally, to one buyer who would likely be a reseller. I am used to taking a loss on gear as it is part of the insanity. I have no interest in packing / shipping any of it. I know I am limiting my market. My pricing seems to be 1/2 of what I see elsewhere. 

I buy high and sell low. Always works. (Well, not financially)