Don't beat yourself up over it. It's not that big a deal and usually passes unnoticed. That's why attention usually isn't brought about.
It is NOT true that anything on the internet is fair game. A lot of what is on the internet is copyrighted. Some people (like Diana Mandache) watermark everything they put on their blogs, archives like the Hesse and others do the same. Some images are in the public domain--usually those before 1935 (?). (I think it's 75 years.) However, when one knows who the specific item belongs to (in this case an ebay seller), those images belong to him/her. It's up to them to make a fuss or not. Copyright is often hard to determine--especially with older items and when they are out on the internet. However, many items are only on the internet because someone broke copyright in the first place. It's part of the reason that you see so many items marked on here. That at least preserves the image somewhat from being passed all around. Eric is right--unless the activate the activa (one example) program that prevents downloading, they are usually able to be saved. Some people prevent that on ebay and other sites though.
Anyway, like mentioned on the other thread, I would look up the thread on copyright. There is some really good information there, esp by Marlene who is pretty much an expert as well as being a librarian and published author.
