I mean if I check using copyscape when buying articles is it going to be fool-proof way of knowing if it was never published on internet? I understand it would not catch those articles who haven't been indexed yet by say Google (within 6 weeks or so). 
Could content checked by Copyscape still be a duplicate?
seo;duplicate content
Foolproof? No. But it does use Google's API so it is getting the largest set of data to compare to which is a good thing. So basically any content that is not blocked to Google's crawlers or has not been indexed yet should be caught with Copyscape.