[NoNan]

I have a collection that I disputed because it's not mine. Ironically, the original collection agent sold the account to another collection agent instead of providing proof of valid debt to me. Again, I disputed to credit bureaus about it, and like before, it always come back as 'verified' and nothing else. Not a single document to prove I am liable for the debt. The wheel keeps turning around and around. This one debt has been reported three time on my credit report and none of Credit Bureau doing anything about it! Is this legal? How can I get this resolved? Thanks