Is a offer of -20 percent on an OBO ad insulting?

20% off is not insulting, but depending on the circumstances it could be naive - which, of course, doesn't warrant a smack.

With that said, last week I put up for sale a very hot item NIB OBO at what would be market price for used ($700). Even given that, two people shot right back with "SOLD for $550. Give me address, I give you $!". This is annoying, regardless of the OBO advert notation because, even given that, these people were low-ballers. Even with that, I told them, thank you, no, and just reminded them of what they were bidding on - which is my way of getting rid of the people trying to swoop in on a price hoping for a newbie scared about spending too much (invariably guys trying to play used equipment dealer for a awhile...)

We do not know how this person asked for the price, or what the model was he was making a counter-offer on, but given his/her tone here, he hardly seems to be one of those guys.

There are lots of asses in the world just looking for a spot, always anonymous, where they can shout at the world. And so it goes...

Again, thanks for all the interesting discussion!
BTW, my offer was worded "I'm interested in your amp, would you consider $XXXX and splitting the shipping?"
I don't have access to the Blue Book prices but his asking price, compared to the new price, did seem low. I know from personal experience, if I'm selling something that I've purchased used, I'm willing and able to sell at a much lower price, of course I didn't know if he was the first or third owner. Most everyone expects and has tolerable amount of loss when reselling an item. It's only through friendly negotiation that a good deal for both buyer and seller may be struck.
FYI, I've just sold a Cinepro 3K6SE ($7000+) for $2499 and I paid the 2nd Day Air, PayPal fees, & auction fees. I guess I'm just easy!
Still looking, Jim

I had a similar situation recently - a pair of speakers was listed for about 25% off retail. I thought that was a little high all things considered - they weren't just a week old or anything - so I offered something around 33% off retail and got a somewhat indignant and insulted no... (but he didn't do antyhing weird, like block audiogon!) I never got around to replying because someone else posted the same set for around 50% off retail and I grabbed them within minutes...

Maybe the seller has his thoughts that the asking price is more than fair, and that the buyer should be educated to know. What the seller fails to take into account is that there are always lots of Krells for sale. So its buyers choice. Seller is a tad upset that he's not getting enough response on his ad, not moving fast enough. If the seller tried to meet him half-way, 10%(take the hit and move on), it would have been a fair deal for both. The seller could easily have ignored his email.

Tweekerman, brings up good point. If a piece is put up for sale with "o b o" then a negotiable sale is implied.