of course NOT. It's like to download music, movies, books. Or use images which do not belong to you on your website. It's a kind of digital theft. I can imagine that it doesn't matter if you make a business or not with your site.
Yes, you cannot buy. You have to participate. And if i understand right, as a beginner you need to post first and create some threads --Maybe one day the algorithms will even evaluate the posts to assign points according to the quality of the content.