What is a lowball offer? How low is low?

Recently I was looking at 2 -3 preamps. One was particularly interesting which was a Conrad Johnson 16LS2. The asking price was $3500. I have always wanted the 16LS2. But it was out of my price range. I couldn't make a serious offer. So I didn't and moved on. A few days later I see that it has sold for $2500! What? That was in my budget. But I would have considered that more an insult than a offer. And because I didn't want to insult the seller, I lost a good preamp to someone who was willing to take the chance. So what is the lesson learned here? Apparently I should insult more sellers . The seller wasn't insulted after all. he sold it for $2500. Or is the lesson that I take this stuff too seriously?Should sellers be insulted when offered 50% of their price?  I had thought that 10-15% below the asking price was a realistic offer. Now I'm not sure. So what do you consider a low ball offer? And please, I understand that a product is worth what someone is willing to pay. So no economic lessons please