Validity is completely orthogonal to utility. It's trivial to craft a broken page with an invisible buy button that validates. I don't know of any major ecommerce site that validates.
Major e-commerce sites will have tested under every major browser anyway, so invalid HTML won't matter so much to them. Smaller shops don't tend to do as much testing, so it's more important that HTML validates.
If it doesn't, it's probably rendering in quirks mode, and you'll have all sorts of problems with the layout if you need to change it.