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

20% is probably not terribly unreasonable. Being a seller myself, I can understand the sellers point of view here. It's exteremely irritating to get many low ball offers in a row. For example, I had speakers listed at 825 for the pair plus shipping. Someone wrote me and said they would give me 500 shipped to their area. I noticed a lot of people don't do research before they make offers. Look at other units similar to what you want and see what they are going for. Then consider an offer.

what a clown...
He cannot block your email, because it was delivered via Audiogon emailer; if he blocks that then he's blocked ALL email from the remailer utility, & therefor will not receive any offers at all. Duh! This seller with no manners whatsoever has just eliminated himself for you, saving you the trouble of finding out after the fact what a jerk he really is. There's no telling what you might have received, very likely *not* in as-advertised condition.
I've received a similar discourteous response at one time & my offer was quite reasonable, esp. compared with the ridiculous asking price which was in fact an OBO listing. I responded back & set him straight. Guess what, no one bought his merchandise & I later saw it advertised on AudioReZoo.

An offer of 20% discount from an OBO-type ad is completely within reason. Everyone reacts differently to offers.
I recently had someone make a lowball offer on some goods (which is perfectly fine), but they also added a lecture on how ridiculous my selling price was. That was uncalled for.
Particularly, due to the fact that the goods sold within four hours at my asking price.
I believe courtesy usually prevails in both buying and selling.

Anyone who is insulted by an offer that is only -20% should ask rock bottom and list as "firm". If you list obo you have to leave some room for negotiation. I have responded with "too low" and I have ignored stupid low offers. Dont think its rude if the offer is obviously ridiculous. Dont take it seriously, feces transpires.

Seller could have added "lowballers will be cheerfully ignored" or "price is firm" or similar text as I have seen many times here on Audiogon.

As a newbie to the 'sport', I have found it isn't always easy knowing what etiquette is in this arena. I for one probably wouldn't know what is fair game for a used Krell, and may have incurred the wrath of this seller too.