I would like to buy some of WP-Script themes and plugins but beforehand I want to test them and see how they would work on my adult site and basically test them. Is it safe to download nulled versions from other places or not?
Using pirated themes is like playing with fire. You might get it for free, but you will end up messing up your site and have it crashed or hacked at some point. If you mean business, then there is the only option for you and that is to get the theme from the original source and you might end up saving yourself time, cash and from headaches. All legit sites offer a refund policy so you will get your money back if you do not like it.