If you’re asking this question, you’re better off buying a finished product. (No insult)
Building it from scratch is not such a challenge, it's more of a time issue. Also, I wasn't sure if you were referring to a kit or literally starting from scratch.
Either way, you sufficiently scared me away from a metal shed but the others are all way out of my price range
Site looked pretty convincing to me. So checked it up.
Yes I'm afraid it's scam