You will find mfrs. claiming this. Fortunately for you -- and unfortunately for them -- they cannot deny warranty coverage because they don't like where you bought the product.
Some keep trying (I think Denon is one), but they seem to fail to understand this practice is illegal in most (all?) of the United States. There is federal law -- the Magnuson–Moss Warranty Act -- that provides a baseline of consumer protection and state laws that often go farther.