A while back I found the same musician's pieces on two different websites. One was providing them under a creative commons license, the other was selling them under their own license, to be used as BGM for other works. I believe in this case it was the musician licensing their music under two licenses.
Still, let's suppose those sites were far sketchier. What is the due diligence I would have to do before relying on the license granted to me by either site?
Being contracted by a license-less third party to perform has been discussed before but I believe this question is distinct from that one.