I think there are competing forces here.
1) There really are folks who seem to offer 30% or so below going rate as soon as the ad comes out.
2) On the other hand, things go fast, particularly good things that are less than $2000.00. This means that if you don't offer quickly, you lose. Plus, the seller doesn't always offer at a going rate price, or doesn't consider condition or age. So I've offered as much as 10% off asking right as a sale goes online. I wouldn't say it was low-balling, but for a product I'm buying based only on reviews, I need to buy for a price that lets me resell for the same price or so.
Separately, when I first started this hobby, I wasn't in a financial position keep pouring as much money as I'd like to into it - this means that I've needed to buy and sell in order to fund my hobby. I would buy things that a) I would like to hear in my system and b) I know I can sell for a few hundred more than I paid. Sometimes, I would see a consignment piece at a local dealer that I knew would go for much more on Audiogon and made a little money that way, too. Nonetheless, I don't consider myself a "flipper" as half of my motivation was to hear the component I would buy in my system, and often I'd find that I'd keep it for years before reselling.
So in short, if you get an offer that's too low in your mind, just "cheerfully ignore" it. Some folks get offended if you ask them to absorb PayPal, some folks price their goods knowing that they'll come down by several hundred dollars....