Some sellers are great to deal with, and some are plain rude.
In my opinion offers that are about 30% off the asking price are fair, and we can negotiate from there (both as a seller and a buyer.) One of the smoothest transactions I've ever had on audiogon was for an item priced at $3300. I offered $2700, seller countered with $3000 and he paid shipping and we had a deal within a few minutes.
Same as a seller. If you make a super low offer I politely decline. No need to get personal. Unless you're a scammer who "purchased" the item I sell and never pay for it, and ask me to send it to another country or your friend who's going to pay me later...