For UK vendors with headshops, such as PlutoPete, then yes it's illegal as they're selling both paraphernalia (ie grinders, pipes) along with other items such as seeds, grow guides etc which is seen in the eyes of the law and was set precedent by the Supreme Court as constituting a implication to consume/use/manufacture a controlled substance so unless Pete fancies taking the matter to the Supreme Court or Parliament, yes it's illegal in the UK at least, which is why a lot of places here a guy will own both the hydroponics and head shop and refer business to each store but they aren't officially the same place, so that's the loophole they work with.