Not everything posted on Usenet is really public. You can download it, but you have to find it. A single upload can be spread across different binary groups. So choose your indexer wisely.
Paying securely can mean different things depending on what and where you are buying.
It's hard to come by a good api or database when it comes to porn / adult content. There is so much stuff out there, you might ask if it is even worth indexing all the content flying around on the internet.