As I read it, he got pissed only after he offered to pay less TWICE. It was after the second offer posted publicly on the Q&A section of sellers the ebay ad.
I think that is a different situation than making counter offers at a swap meet in person. The Ebay Q&A inquiries can can be totally from right field but effect other potential buyers. At a swap meet, you have your exchange of offers and it's done and you move on. On ebay, the exchange is recorded. So it's like you are in that guys swap spot heckling him all day.
The first offer wasn't the problem, it was the second from what I can tell.