Are you getting ridiculous offers from new members

I think that's just the nature of dealing with the public.  Back in the late 70's when I sold high-end equipment at a brick-and-mortar retail shop, same sort of thing would happen back then.
The worst Audiogon experience of this type was when a guy who lived about an hour's drive from my home said he insisted to pay me cash, we talked and came to an agreement on the price, when he arrived at my home he said he thought about it and changed his "offer" reducing it by $600.  My takeaway from that was that he was attempting to circumvent the process of making a formal offer on Audiogon, so there would be no evidence, so when someone uses the Audiogon email system to negotiate the price, I make them make a formal offer.  Using that method, I still get the "low ballers", but haven't had a real sale that wouldn't go thru.

So, did you sell the item for his $600.00 reduced offer?

Ask if "they" would be willing to sell one of "their" items to a lowball offer.

If your selling, and we all have, and have a set what we believe is a fair price, then that’s our opinion.

Buying used is just that and a buyer does not know the circumstance behind the sale, maybe your looking to unload, maybe your holding tight, maybe they thought it was something less then what it is and if that’s the case then does it make sense “not” to include the make an offer option?

In the end, Money was spent buying, selling and if resold buying again. Money does not grow on trees, and generally people really want the best deal for themselves.

Shrug it and move on, there’s a buyer for everything. These statements by no means accept rude or inappropriate behavior on either side of the coin. 

We all haven't , there are more folks on here than you would think that give away gear , not all cheap stuff either .