Yes it's legal - Halfords for example does this a lot with car seats. They are delivered direct from manufacturers. Normally this works with what's called web services where one database will be queried live (the manufacturer) to see if it's in stock and this info relayed back to the website and if it's not there you can't order it.
Not always. There is usually a description of delivery terms on the website. Halfords do tell you. Comet/Currys don't tend to. In fact even in their stores they often don't have it 'in stock' and have to order it from the manufacturer which normally takes about one extra day.