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

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. 

@yesiam_a_pirate ,

An excellent point. Different cultures have different bargaining techniques, didn't consider that, myself.

Appreciate you thinking on a more cosmopolitan level.

@thecarpathian what he said politely is that some of us have class. 

Ignore it or send a note of appreciation but no interest at the price they are willing to bid especially if in excellent/mint condition.  I do this all the time and have sold many pieces over many years.  Also the variable of how long you want to wait to sell the piece has to be considered.  If they want it they will let you know.

Years ago abroad, a few of us were swapping stories of bargaining and my friend David told me of an attempt to buy a rug in a rug shop in Cairo. He offered what he thought was a well-crafted low-ball offer, in hopes of moving to an acceptable place in the middle.

The owner looked startled, grabbed David by his shirt and rushed him to the door and shoved him into the dusty street. He slammed the door shut and flipped the sign to "closed".

We agreed that this was the most spectacular fail at negotiating we could recall.