It may be bad form, but as an honest businessman, I'd have no issues with someone knowing what I offered for the rifle. Just because it's a fair deal, don't mean the guy can't get more for it.
Don't get me wrong, I wouldn't want people thinking they could do business like this all the time, but I wouldn't get THAT upset about it. You'd have to be a fool to think that it's not happening behind your back.
