For something to be insulting, it must be personnel. A person that Im not familiar with, cannot insult me. I do sales for a living. Its not personnel, its a business. By listing equipment for sale, the seller is asking for offers. Most of the offers wont match the sellers expectations. That, is by definition, how free markets work. If the seller doesnt like the offer he wont accept. As a buyer, if you make a lower than asking price offer, it may not be accepted.
What I like about A-Gon is that you can talk to people and determine what type of person you are dealing with. That is not always available on Ebay.. You may have gotten a clue about this person by their reaction to your offer. In sales, a simple no will always work. Why get upset?
Furthermore as a seller, there is no obligation to deal with unqualified buyers. What I mean, is that it is OK for a seller to only deal with high probability prospects. Sellers are under no obligation to answer all the questions that they are posed. I will qualify people by asking them, If I answer all your questions to your satisfaction, are you prepared to spend $XXX. It is an easy question to find qualified buyers. I learned the hard way, spending too much time with people who were not buyers.
Have fun and good listening.