Most sellers on Audiogon take care of their investments and they are not usually your brick-and-mortar brands. With that being said, my rule of "low ball offers" are 15% -20% on 2nd market value whether I'm the buyer or seller. You can always check the "sold and complete" sales of an item on the other website (not the asking or selling price).
There are too many people thinking that they can get flea market deals on fine audio equipment. I would recommend purchasing your stuff from a Fleamarket if you want "low balled" price. But on the other hand, if someone is listing an item at a great price, then jump on that purchase while you can.